If you overclocked it and it used to work but now no longer works with the same overclock, then it means something is no longer able to keep up with the signal somewhere, possibly due to noise on the power supply, aging of PLL components or other factors.
If you haven't tried it already, reduce your overclock refresh rate. There isn't much else you can do short of popping the cover off and poking around the circuitry but that is something only an advanced electronics hobbyist/tech or engineer is likely to be successful at messing around with unless the problem is physically obvious.