What version of Workbench do you have installed, what version of Cybergrafix, have you cleaned and reseated everything, pressed down on any socketed chips, removed any patchs or hacks that could be interfering, could it be heat related, have you flipped your fan around to the right way (so that it's pulling air out of the case - a common problem with 4000's is that they're installed backwards), how's the condition of your caps and battery, etc. etc.
Honestly it sounds like a bad memory chip on the card, to me, but the above list is just a few of the things you could start by checking. Good luck!
