There are many reasons why this could be happening. First of all you need to determine whether the problem is with the reading of the data from the disc or if it's a playback issue
If it is a problem with the reading of the discs there should be some tell-tale signs. For example the drive could be making quite a lot of noise, the drive would tend to keep changing speed and the read light would flash more erraticallly. To be honest it doesn't sound like the problem is here though
To try and figure where the problem is with playback you need to look at several things. Has the playback always been like this or has it just recently started to degrade in performance? What DVD playback software are you using? Is it the latest release? Have you tried any other software? Whats your CPU, memory and graphics card like? Presumably it meets the minimum specs for DVD playback, probabbly about a P2 400Mhz at least 128mb ram and a 16/32mb graphics card