Local Channels display in black and white

I have 4 recievers and only one of them does this. All my local channels when tuned to after about 3 minutes the nice clear color picture starts flickering from color to black and white. If you change the channel and go back to what you were watching its color again for about 3 minutes then starts flickering. If you don't change the channel the flicker turns into completely going black and then gray and then completely black. I've tried resetting it, I've tried unplugging it for a while and plugging it back in . It is hooked up to a brand new flat screen and only does on the local channels so I know its not the TV that has a problem. Help

Kimberly