Lately I noticed that all of my gameboy cartridges seem to be loose in the cartridge slots. Not a lot, just wobbling 1/2 a mm or so, but enough to let a game freeze. This isn't a problem with (back then) new Game Boys, but the contacts of the slot lose their tension and the circuits are interrupted for milliseconds.

The Solution is very simple: insert a thicker business card right under the cartridge (smaller side) when setting it in. It fixes the cart enough to not wiggle anymore at all.

Problem solved