What characteristics of the multiplier and modulator provide large periods?

Don't bother with looking for a large period (though Knuth spends about half a book on pseudorandom number generators). The problem is that the sequence is predictable. Given ten sequential values from anywhere in the sequence, I think there are algorithms that will determine the sequence. This is from dim recall of some Crypto '90 presentations. Perhaps someone has the papers in front of them, or can re-derive the results? Given plaintext XOR'd with a sequence, you can make pretty good guesses at ten values in the sequence, and if you have to try a few thousand guesses, it will still only take minutes or hours to crack. John