let me guess, your running ibrowse as webbrowser? for me, if ibrowse is running, expect a crash every 30 minutes, although i noticed, if images are disabled it stays up somewhot longer, maybe something around 2-3 hours. as soon as i run a freshly boot up amiga without ibrowse (or aweb, which is only slightly more stable than ibrowse) i usually switch my computer off (yeah, you have heared right ;-) ), no crashes at all. record there should be last xzentrix (maybe topped by next xzentrix in a few weeks
http://www.xzentrix.de ) where it was running about 20 hours or so playing games with whdload. no reboots, just normal exit via whdload hotkey.