2000 cd player acting up

[KnoX78]

The cd player in my 2000 Xterra has started skipping when I try to play 'burned' cds. It still plays regular cds perfectly, but with burned ones the sound is sort of fuzzy and then it skips abruptly before resuming play...it then repeats the cycle at no regular rate. Any ideas on what might be happening? I'm going to try cleaning it, but it seems to me something worse might be going on.
Also, if I were to replace the head unit with an aftermarket (sony, etc), are there any irregularities I should know about? I read one thread that mentioned stock speakers have 2 oHm impedence, which might cause replacement problems.
Thanks in advance for any help.