There could be a loose wire inside the unit, or inside one of the plugs. The result would be that everything looks and feels like it's plugged in tight but in reality there's a broken wire inside either the screen on the USB cable.
It might be okay when it's cold but when warm it might expand just enough not to make the connection anymore.
The first place I'd look is any part of the USB cable that bends sharply or runs underneath something that moves. (Like you're seats for example) If you find a kink in the wire or the outer casing of the wire is broken it's a real good chance that you've got a very small cut in one of the wires.
Try replacing the USB extension cable first if you've got one. That's a quick check. If your monito plugs directly into the computer without an extension then just go over it the entire length of it carefully. If you find nothing open teh screen up and see if perhaps a wire came loose in there, perhaps when you swapped the touch screen out previously.