I've seen something similar happen when you have a circular reference in a specific way in a class. It's trying to show intellisense and ends up in an infinite loop trying to build the list. I don't remember the exact circumstances now, though.
Use Process Monitor (http://technet.microsoft.com/en-us/sysinternals/bb896645.aspx) and trace the activity of Visual Studio when it exhibits this behavior. I have done this many times before with great success.
I am actually monitoring Excel on startup to determine why Excel is having problems with a VSTO Add-in. It turns out Excel is trying to also load a previous version, so I have two add-ins loaded. I figured this out in less that a minute. Without Process Explorer I might have been troubleshooting for hours or days.
Given up and wiped VS 2008 & all supporting software. Reinstalling now. It is fun to debug, but I had to cancel a meeting this morning because I couldn't get anything going.
If I get a job at MS then I'll look into it further... but this one is just too hard for me and my limp debugging skills leftover from years doing .net ;)
DOH! Reinstalled and it is still broke! I may actually have to solve this :(
@amissico: thanks, i had forgotten about that completely.
The problem is somewhere in the parsing thread, and I can't work out how to pinpoint what it is doing wrong. I see lots of FS activity, but nothing that I can identify as wierd. It just keeps building temp files (all the same name?) It just looks like it is doing everything everytime, the slow way. I don't know what it should look like, though.
Startup is fine, this is mostly happening when I edit files. It just starts chewing for intellisense, it appears, but why it takes so long is wierd.