1983 Other GMC Models air bubble?

hi,i just put an aluminum radiator in my caballero.it has a crate motor-ram jet 350.i also put in a new 180 thermostat.i seem to have an air bubble in the motor,as the temp goes from 170 to 200 as i drive around my block.it goes up and down 3 time every half mile.i check the rad temp w/a laser meter it was 100 degrees. only the motor temp is going up and down.does that sound like air and how do i get it out? thanks,steve