The drive belt is losing elasticity. You will need to buy a replacement belt (worst case scenario).
Alternatively you can try this:
1) Take the drive belt out (black rubber band-like belt)
2) Put it in a cup of boiling water for 1 minute
3) Take the drive belt out and immediately immerse it into ice cold water. Remove it and pat it dry with a paper towel
4) Before you put everything back together, put some masking tape on the plastic disc on the inside of the top of the dvd drive cover. This plastic disc mates with the spindle/motor which spins your discs.
The tape will weaken the pull from the magnet. The hot/cold treatment will make the drive belt shrink a little bit and give you slightly more tension. Doing this + the masking tape works 9/10 times.
UPDATE: I've found a suitable replacement for the drive belts--orthodontic bands. Get the medium sized ones which will give you about 6 oz of pull/resistance.
You can get them from a dentist or buy them at the pharmacy. They cost $2-3 for 100 bands.