Precision T7500 - USB Flash Drive Buffer

Hi I have a Dell Precision T7500 running updated Windows 7 64 bit. When using any USB flash drive (large or small and any type) I am having problems playing back my videos from the actual drive. The picture starts and runs ok for 30 seconds then freezes for a short time (30 seconds) then runs fast then freezes again. It doesn't matter if I am using my USB3 card (super fast) or the integral USB2 the same happens. I have a new large memory gaming type video card and my previous video card smaller. It happened with both cards. I have twin Xeon x5670 cpu 2.93GHz and 48GB ram. Have updated all drivers at Dell Support site. I suspect the problem might be a buffer size problem but was not solved when I increased the buffer of my machine. Barry Clutterbuck