DVD skips back to the start - how to pinpoint problem source

[niemie]

Ok, I have backed up about 12 of my DVD's. 3 of them have skipped and started over at some point in the movie. I am trying to nail down the cause of this. I am using DVD Decrypter to rip, DVD Shrink to compress if needed and it auto launches Decrypter to burn.

My Specs:

P4 2.8
1 Gig Corsair XMS RAM
40 GB Sata Raptor HD
Sony DRU530-A DVD burner
Standalone DVD player = Sony DVP NS700P

I have used a mix so far of 10 8x Memorex DVD+R's and 2 4X TDK DVD+R's (Taiwan if it matters) - so far only the Memorex's have done this so it could be media but I am not far enough into it to know.

Does having other programs open when you are ripping/burning matter? Also, I have been defraging after every 3rd or 4th movie because I delete all the files off my HD as soon as I am done with each movie.

[niemie]

a possible answer? Could it be that burning at 4x is more reliable than burning at 8x?