This is about the fastest random number generator you can get although it is really just a sequence of 256 numbers using 14 bytes (code and data if seed is in ZP)

An .ssd is attached and you can see the description here http://www.retrosoftware.co.uk/wiki/index.php/Random_number_generator_%28very_fast_-_very_basic%29

That's actually a pretty good algorithm. If you weren't worried about reproducibility, you could EOR with a timer at the end, e.g. EOR &FE44, which should add a bit more randomness!

I was tempted to self mod the addition every time the seed gets to 0 to chain many different sequences but the timer is definitely in the spirit of this code.

As a little side note to newer programmers, it is very useful to keep separate random numbers for game/sound/graphics/etc (especially in multi-machine synchronised state games) to aid in debugging and at the cost of a few bytes, well worth it.

When debugging, I would recommend not using Rich's excellent tip (of EORing with the timer low-byte) or NOPping it out in a binary to make bug reproduction easier.