2002 monte carlo overheating on and off

Soooo, 2002 monte carlo nothing special about it. have replaced thermostat, had the
system pressure checked, and flushed. It has been bled to check for air a ton of
times. The heat stops and then the temperature will start to go up sometimes all the
way up to the red sometimes it will go all the way up to 3/4 then suddenly right before I
pull over drop back down and heat comes on again. any ideas i am at my wits end and
have spent 600 and car still does this