Some hints:
RF AND composite problem? Problably U30 fried, or something around it (check the underside of the MoBo too).
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Guru on soft-reboot: did you dismount some RAM-disk before? Could be some RAM-related soft making a mess.
Download SnoopDOS, put it very soon on Startup-sequence, setted to save the log on an archive, open the archive when the Amiga comes back to live again.
If not a software-related problem, them u have to change ALL electrolytic caps on the board. They are few, unexpensive and easy to replace. Ripple on +5 rail is one of the most common cause to hardware issues.
Other thing could cause this: ur PSU. Open it and look the caps. Again: they are few and (much more) easy to change.
