I own a Commodore 64, and when I use it for a long time—mostly in the summer—funny-looking waves appear on the screen and scroll downward. After that, the waves get bigger and bigger, the computer starts printing characters all over the screen, and the keyboard won't operate. Is there any way to stop these annoying waves?
It sounds like a classic case of overheating. Computer chips are designed to operate within a specified range of temperatures. For example, the VIC-II video chip in your 64 is rated to function normally between 32° and 158°F (0°–70°C). At the high end of their rated ranges, chips can start acting strangely, and if a particular chip isn't quite up to specs, the bizarre behavior can begin to show up at lower temperatures. While it's unlikely that your room temperature is reaching 150°, it could get that hot inside the computer's plastic housing, since all chips emit heat as they operate.
There are a couple of possible solutions. First, make sure the ventilation slots on the underside of the computer and the expansion slots on the back panel aren't obstructed. If that's no problem, perhaps you can set up a table fan to keep air circulating over the computer on hot summer days (it'll help keep you cool, too).
Still no results? A more drastic solution is to remove the foil shell which covers the circuit boards of newer 64s. The foil is designed to reduce RF (Radio Frequency) interference, but it also traps heat. Carefully remove the foil shell and see if this solves the problem. (Unfortunately, removing the foil voids your warranty and may also cause more video interference with nearby TV sets.)
Another alternative is to have your computer checked out by a qualified service technician. Perhaps a slightly defective chip is responsible for the overheating.