1) FP1 is out...but that an entirely different animal than 8.5.1. 8.5.1 is a notable release, while FP1 is just a bunch of fixes piled onto 8.5.0.
2) If you have support, open up a call. Just because you can't produce it upon demand doesn't mean IBM support won't have some magical debugging parm that may help track it down when it happens the next time.
3) Not knowing anything, what you're describing sounds like some sort of semaphore problem. The client is patiently waiting (while you are IMpatiently waiting...) for something to transpire. It either times out (networking issue?) or finally comes back and completes. Cranking up some sort of semaphore debugging or opening the console window and watching could be helpful the next time it happens (you could put "Debug_Console=1" in Notes.ini to cause the a console window to appear when you start the client--it may shed light on what craziness it happening in the background).
Feedback response number WEBB7V8TED created by Rob Stitt on 08/24/2009