Engine heating and cooling

My 1992 F-150 302 v8 4x4 auto trans is having a problem keeping the thermostat were it should be. I just put a new one in and when I start driving it goes up to the normal line then goes straight back to the cold. If I leave it idle it will raise up to n or o. When I'm driving it will come down just above the normal line and when I brake or slow down it goes below to the c. The old thermostat and the new do the same thing. What am I doing wrong. Thanks