WCF supports service throttling. This allows you to limit how many messages/connections/instances that can run at the same time. Search for ServiceThrottle on MSDN.
You specify your ServiceThrottle in app config like so:
<behaviors>
<serviceBehaviors>
<behavior name="Throttled">
<serviceThrottling
maxConcurrentCalls="1"
maxConcurrentSessions="2200"
maxConcurrentInstances="1"
/>
</behavior>
</serviceBehaviors>
</behaviors>
If you suspect throttling is the problem, look for the ServiceThrottle object in the debugger, or in a dump. Here's an example using cdb on a dump:
Dump ServiceHost
0:000> !do 0000000001647fc0
Name: System.ServiceModel.ServiceHost
MethodTable: 000007feef98a7f0
EEClass: 000007feef352d60
Size: 248(0xf8) bytes
(D:\Windows\assembly\GAC_MSIL\System.ServiceModel\3.0.0.0__b77a5c561934e089\System.ServiceModel.dll)
Fields:
MT Field Offset Type VT Attr Value Name
000007fef23d5918 400098a 4c System.Boolean 1 instance 0 aborted
000007fef23d5918 400098b 4d System.Boolean 1 instance 0 closeCalled
000007feef93aa00 400098c 8 ...ct+ExceptionQueue 0 instance 0000000000000000 exceptionQueue
000007fef23d6038 400098d 10 System.Object 0 instance 0000000001648188 mutex
000007fef23d5918 400098e 4e System.Boolean 1 instance 0 onClosingCalled
000007fef23d5918 400098f 4f System.Boolean 1 instance 0 onClosedCalled
000007fef23d5918 4000990 50 System.Boolean 1 instance 1 onOpeningCalled
000007fef23d5918 4000991 51 System.Boolean 1 instance 1 onOpenedCalled
000007fef23d5918 4000992 52 System.Boolean 1 instance 0 raisedClosed
000007fef23d5918 4000993 53 System.Boolean 1 instance 0 raisedClosing
000007fef23d5918 4000994 54 System.Boolean 1 instance 0 raisedFaulted
000007fef23d5918 4000995 55 System.Boolean 1 instance 1 traceOpenAndClose
000007fef23d6038 4000996 18 System.Object 0 instance 0000000001647fc0 eventSender
000007feef929a60 4000997 48 System.Int32 1 instance 2 state
000007fef23ce158 4000998 20 System.EventHandler 0 instance 0000000000000000 Closed
000007fef23ce158 4000999 28 System.EventHandler 0 instance 0000000000000000 Closing
000007fef23ce158 400099a 30 System.EventHandler 0 instance 0000000001648660 Faulted
000007fef23ce158 400099b 38 System.EventHandler 0 instance 0000000000000000 Opened
000007fef23ce158 400099c 40 System.EventHandler 0 instance 0000000000000000 Opening
000007fef23d5918 4002e78 56 System.Boolean 1 instance 1 initializeDescriptionHasFinished
000007feef981300 4002e79 58 ...meKeyedCollection 0 instance 00000000016481a0 baseAddresses
000007feef981500 4002e7a 60 ...patcherCollection 0 instance 0000000001648220 channelDispatchers
000007fef2416b80 4002e7b c0 System.TimeSpan 1 instance 0000000001648080 closeTimeout
000007feef980e70 4002e7c 68 ...erviceDescription 0 instance 0000000001656e28 description
0000000000000000 4002e7d 70 0 instance 0000000001648290 extensions
0000000000000000 4002e7e 78 0 instance 0000000000000000 externalBaseAddresses
0000000000000000 4002e7f 80 0 instance 000000000166cf80 implementedContracts
000007feef989770 4002e80 88 ...nceContextManager 0 instance 0000000001648300 instances
000007fef2416b80 4002e81 c8 System.TimeSpan 1 instance 0000000001648088 openTimeout
000007feef8e9890 4002e82 90 ...rformanceCounters 0 instance 0000000000000000 servicePerformanceCounters
000007feef98b708 4002e83 98 ...rformanceCounters 0 instance 000000000171c590 defaultPerformanceCounters
000007feef981a18 4002e84 a0 ...r.ServiceThrottle 0 instance 0000000001648340 serviceThrottle
000007feef980c20 4002e85 a8 ...erviceCredentials 0 instance 0000000001800ed0 readOnlyCredentials
000007feef980d58 4002e86 b0 ...orizationBehavior 0 instance 0000000001801240 readOnlyAuthorization
0000000000000000 4002e87 b8 0 instance 0000000000000000 UnknownMessageReceived
000007fef53bc640 4002e77 f80 System.Uri 0 static 0000000001648110 EmptyUri
000007fef23d6038 40032cc d0 System.Object 0 instance 00000000015be070 singletonInstance
000007fef23d8420 40032cd d8 System.Type 0 instance 00000000016486a0 serviceType
000007feef981ee0 40032ce e0 ...ontractCollection 0 instance 0000000001657468 reflectedContracts
000007fef23d1000 40032cf e8 System.IDisposable 0 instance 0000000000000000 disposableInstance
Dump Throttling Info for the ServiceHost
0:000> !do 0000000001648340
Name: System.ServiceModel.Dispatcher.ServiceThrottle
MethodTable: 000007feef981a18
EEClass: 000007feef34c2d8
Size: 72(0x48) bytes
(D:\Windows\assembly\GAC_MSIL\System.ServiceModel\3.0.0.0__b77a5c561934e089\System.ServiceModel.dll)
Fields:
MT Field Offset Type VT Attr Value Name
000007feef981a90 400371d 8 ...cher.FlowThrottle 0 instance 00000000016484a8 calls
000007feef981a90 400371e 10 ...cher.FlowThrottle 0 instance 00000000016485d8 sessions
000007feef902140 400371f 18 ...her.QuotaThrottle 0 instance 0000000000000000 dynamic
000007feef981a90 4003720 20 ...cher.FlowThrottle 0 instance 0000000001779f80 instanceContexts
000007feef9808c8 4003721 28 ...l.ServiceHostBase 0 instance 0000000001647fc0 host
000007fef23d5918 4003722 38 System.Boolean 1 instance 1 isActive
000007fef23d6038 4003723 30 System.Object 0 instance 0000000001648410 thisLock
Dump Session throttling information
0:000> !do 00000000016485d8
Name: System.ServiceModel.Dispatcher.FlowThrottle
MethodTable: 000007feef981a90
EEClass: 000007feef34c390
Size: 64(0x40) bytes
(D:\Windows\assembly\GAC_MSIL\System.ServiceModel\3.0.0.0__b77a5c561934e089\System.ServiceModel.dll)
Fields:
MT Field Offset Type VT Attr Value Name
000007fef23dd9b8 40034f5 30 System.Int32 1 instance 2220 capacity
000007fef23dd9b8 40034f6 34 System.Int32 1 instance **2220 count
000007fef23d6038 40034f7 8 System.Object 0 instance 0000000001648618 mutex
000007fef23c4d70 40034f8 10 ...ding.WaitCallback 0 instance 0000000001648598 release
0000000000000000 40034f9 18 0 instance 0000000001648630 waiters
000007fef23d6748 40034fa 20 System.String 0 instance 00000000016483c8 propertyName
000007fef23d6748 40034fb 28 System.String 0 instance 0000000001648550 configName
0:000>