I broke the belt on my 90 chevy pu the temp did go up high now the temp gage goes back and forth from about 155 to 200 as I drive the pu what is wrong with it?

It is possible that when it heated up a little too high when the belt broke that it screwed up the thermostat a bit and it might be sticking a bit now. I would change the thermostat and see if that helps and dpeending on how the rad looks inside, possibly flush the cooling system first.