Have you checked to see what happens with those videos when you watch them on a PC?
Also, what's the firmware version of your TV?
I have a Samsung 55" Series 750.
This problem has just started. When watching an XVID-AVI movie through the USB port, the action becomes "jumpy and distorted at the 27:30 mark. This happens on all movies of this type.
The set is about 2 years old. Everything worked fine until last month.