monitor problem

bought a Samsung LE32A436T1D Series 4 32" HD TV + Freeview 1366x768 10000:1 1080i 3xHDMI tv and am currently using it for a pc monitor. I have it hooked up with a vga cable, and my graphic card is a ati 4870 512 mb.

My problem is that sometimes the picture goes on screen, such as when i want to bring up task manager, or when i go on a game, or loading screens between levels etc... the sound is still there, and the program or game its self is still runnign fine but to get the picture back i have to either turn my tv off and back on, or go through the tv source till i reach the pc one again... any ideas?