i often leave my system run overnight and i always have an owb running with always 10+ tabs opened and never ever experienced such an issue (and im famous to have weird issues :)
what i could think of is to replace your whole mossys: with an original from the latest iso (you could even check the dirs!files by hand for any difference but i guess it would take more time than an overwrite).
also check for every 3rd party component in your system esp. regarding to ambient and mui then disable/remove them one by one to find the guilty (if theres any). look for the famous problematic mcc classes to see if you might have a not suitable one (afaik the latest ones from the xy.mcc Open Source Team are included, but i havent touched any mcc stuff since a long time ago since i've got a said-to-be-reliable-and-working mcc pack from a trusted dev, so i dont remember exactly what could cause problems (maybe nllist and co.?).
you can always run memtest to check if your ram is ok, just to be sure.
DEAD pegII/G4@1000.1gb ram.radeon 9200pro
240 gigz hd.nec dvdrw.MorphOS 2.4 DEAD
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amiga1200T.blizzardppc@180/040@25.96megz ram
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zx.spectrum@3.5