The only things that usually "degrade" in a gameboy cartridge during a normal lifetime are the battery and the electrical contacts. The battery will eventually die causing a loss of all save data and the ability to save (the battery is apparently replaceable). The contacts can also corrode, which can make the game temporarily unplayable.
Nothing happens to the Random Number Generator, which is what you're describing here. Crystal, like almost all videogames, uses a pseudograndom number generator, and sometimes quirks in how numbers are generated appear; this can have effects like certain moves are more likely to take effect on certain turns in the battle due to certain predetermined sequences of numbers. If you're seeing anything at all, odds are it's a quirk of the PRNG.
Sometimes games have long, preset strings of pseudorandom numbers that are generated at certain times like when the machine is turned on; in many early games (like the Pokemon Trading Card game) this is noticable if you savestate and repeat an action; "random" actions like coin flips will always occur in the same sequence if you restart from the same savestate.