First off, the computer DOES have a mini compression issue for playing back video in REAL TIME on a computer monitor which shows video differently than your tv or your lcd of your camera. It's going to look jerky,broken up and too fast at times. But thats just part of the 'digital process' of viewing on your computer. Its fine. Don't panic.
If you have firewire from your camera to computer, switch to that for video transfer---it's faster, cleaner.
And then there's your editing/hard drive storage. The quality of your editing program and the compression it uses also effects the playback of the video.........go into your 'preferences' under program and you can change the quality of the compression and playback and TRANSFER.