YES! I am also experiencing this from FMSA8 running on Win2003.
You're the first person I've found who is also experiencing this - it's a relief to know it's not just me.
It appears that it's somehow related to cookies, but I haven't established exactly what that relationship is. Our web environment makes it easy for our users to collect a fair number of cookies, and it seems that deleting some subset of the cookies the browser is holding onto clears the problem up (temporarily), but there isn't any one particular cookie that is the culprit. Our next theory was that it was the NUMBER of domain cookies that the browser was holding, but I don't think that's the case either. My next guess is that perhaps IWP freaks out if the AMOUNT OF COOKIE DATA exceeds a certain size. I plan to test this by creating a test page that gives the browser several large cookies to see if I can induce the problem.
Knowing Filemaker Inc., I wouldn't be surprised if this is a known issue (which they're keeping under their hat) which is fixed in FMSA9 and the only way to get the fix is to pay them more $$$. (As was the case with the browser security issue with IWP under FM6.)
PLEASE REPLY IF YOU'VE RESOLVED THIS ISSUE!!! Thanks.