Hey there,
Sorry to hear that the crashes have resumed. It's awful to have to deal with it.
Assuming all your Amigas have the same processor, here's what I'd do:
First, check to make sure the program's being started with plenty of stack space.
Delete the iBrowse disk cache and set the size for disk caching to off.
If those don't help, copy the MUI:Libs/MUI/ and Libs: directories over to the flakey Amigas. (Again, assuming they're all the same processor type.)
If that doesn't solve anything, run CyberGuard (or whatever's appropriate for your processor), SegTracker and Sashimi and see if that doesn't help identify the culprit.
If you've got different processor types in each of the Amigas, it'll take a little more time and effort. You could spend all day combing over your libs and not find any old/incorrect ones. I'd probably skip right to the debugging programs and see what turns up.
If absolutely none of this helps, I'd back up my SYS: directories and start from scratch with a minimal install. If you get to the point where you can get on the net and launch iBrowse, and the crashes stop, then I'd copy a directory at a time over from the backup and try to isolate whatever may be causing the instability.
If the crashes persist after all that, it's time to start looking at the hardware.
...anyway, that's what I'd do. Hopefully you'll find something that at least points you in the right direction.