I'd try things in this orded... pushing down firmly on all chips, testing other gfxdrivers for the gfxcard, test machine without gfx drivers att all, removing the gfx card, remove the new ZIP RAM just installed.
If nothing of above does anything then while the machine is as above I'd install OS3.1 and see if that works with and without gfxcard and different drivers, if not I'd clean the system from the gfx drivers and card and change back an old chip one at a time (with the rom last) and check if it works untill I find the new chip that is faulty or have gone through em all without any change in witch case I would probably give up and send it for repairs. :/
