iPhone app hangs in [NSString drawInRect:withFont:lineBreakMode: alignment:]

Question

I'm using the following function:

CGSize s= [string drawInRect:CGRectMake(0, 0, w, h) withFont:font lineBreakMode:UILineBreakModeWordWrap alignment:UITextAlignmentCenter];

string is a NSString and when this method is called - the iPhone is hangs for some time. I've set good values for the parameters. For a test, I wrote a loop with 99 iterations and call only this method inside loop. It hangs on iteration 15, and some others.

When I remove the loop in my App it hangs so rarely that is practically impossible to catch as it appears at the most inopportune moments.

I have no idea. :(