You say it "frequently" has this problem. Does it randomly come and go, or are there particular conditions that seem to provoke it? For example, does it work when cold, then get worse as it heats up? Or does banging on the unit have an effect? Things like that can provide clues as to cause.

In the absence of any information other than "it doesn't work sometimes," all one can say is that you have more investigation to do. First, perform a careful visual inspection of all relevant connectors and related circuitry. Then check things like the red video signal line. If you don't have the instrumentation to do so directly, swap red with another color (blue) and see if the problem moves.

That might do for a start. Others will soon chime in with their suggestions, if history is any guide.