Late model C64's really liked to fry their PLA chips. These were to the C64 what a core logic component like a northbridge is on today's machines. Originally a PLS100 chip was used which, while being the most unreliable chip in the machine aside from RAM, at least usually lasted a reasonably long amount of time. Later, Commodore started making their own PLA chips in-house, and these were of a significantly lower quality. All of the C64 chips were 5V TTL with no heatsinks, which didn't bode well for general reliability. Commodore got a little smarter with the C128 and started shipping with internal shields that doubled as heatsinks.