CD & DVD – similar problems

windows xp/ cd r/w/ dvd rom/ 60g hd/ 256ram

2 Problems:

1- when i go to record music to the cd rom, i can record several songs .. but if i stop and want to go back to it at a later time, it says the disk is full, and i cannot record anything else, unless i erase the entire disk and start over.
2- when i play a dvd, it starts out fine … but after a short while, the video slows and gets behind the audio. sometimes the picture stops and goes … but the audio mostly continues.

It sounds like a buffer/cache problem to me.. but i’m not sure. Any suggestions.. and if so, please tell me how to do what you suggest. I’m a fairly new tech and still learning.
Thanks…
Nancy