okay, so, awhile back ago my lower intake manifold started leaking coolant. and it started running hot, so I replaced the gaskets, then rechecked, still ran hot, so I replaced the thermostat. still ran hot (180 tstat) I replaced the cap, made a little difference. it will run cold for awhile, then just jump up, then go down then all the way up then back down again. then the coolant level gets low. probally from overheating to the overflow. will a bad heater core cause this? I can feel the heat come and go with the heater on as the thermostat opens and closes. any help, please, ive run out of ideas, and there is no coolant in my oil, (head gasket)