when the vf 3 series we have runs at any rpm range the screen jumps around... any idea what may cause this??
Yes, maybe; is this a machine with the old amber CRT monitor?
If the answer is yes take the back cover off, the video card is near the bottom laying horizontally, with the machine running and the screen jumping around carefully put a bit of pressure upwards on the end of the card...the jumping should stop - at least it did on our HL1 with the same problem. Wedge a piece of firm rubber foam under where you had your finger and put the cover back on.
If this is not clear enough I can put up a picture.
By the way be careful poking your fingers around inside the monitor housing and make sure you only touch non-conductors.