I agree, sounds like a RAM issue to me too. But could simply be a bad RAM slot or bad contact. Obvious thing to do would be to remove all RAM chips/modules and add them back one by one (or pair by pair), testing each time. Also, try and clean the contacts on the RAM modules (or legs on the chips) and the sockets with alcohol.
Clean every connector inside your Amiga (i.e. power pin connectors, accelerator edge connectors, etc.).
