Snow on screen/wiggle the coax

I have a p52830bl. Once I turn the tv on it works fine then after about 5 min the picture turns to snow with some of the programing still viewable. I have "wiggleled" the coax cable and the screen will clear up then go back to snow 15 min later. (pain when trying to watch a 2 hr movie) I have tighted all the connections in the back and it still doesnt help. I just have to keep movinng the cable in the rear of the tv to get the picture to clear. I also hooked the coax to a diffrent tv, thinking it was a bad cable however the cable works fine. Can I replace the board and where do I buy one? Also where can I buy a reasonably priced screen? thanks, lost in winter wonderland.