Video play back Jerky

[Wingham]

I have a Sony VAIO VPCCA3C5E. When playing back 'AVI or 'mp4' files the play back is jerky, stop / starting. However it only does it on a few files and not others. I try the same files on my lower spec ACER laptop they are all fine. However, if I play the same file from a external hard drive via USB they play perfectly.

It's a top end Laptop with 8GB ram,  i7 proccesser, 500GB HD and a the best possible video card so power shouldn't be a problem

I have tried:-

Switching off the anti virus and uninstalling it just in case

Updating the video drivers

uninstalling and reinstalling the video software (Media Player Classic)

Hard drive clean up (only 60% used)

Hard Drive Defrag

Check for driver updates for the HD on the sony web site.

Does it sound like a hard drive failure is on it's way?

Any ideas or tips would be great!