Blank pages served by IWP?

[gumpish]

Hello.

I have Server 8.5 Advanced installed on a Win2003 box. Everything works fine except that once in a while web forms served via IWP will come up as blank pages. I haven't been able to pin down certain facts (so far the only browser I KNOW has the problem is Firefox, both on Linux and Windows XP), and clearing the cache appears to resolve the issue... (but I haven't really established causation there either)

Anyone heard of this? It's annoying because I can't reproduct the problem on demand, so I doubt FileMaker, Inc. will help me much if I call them and spend money on an incident. I couldn't find anything on their knowledge base.