99 pontiac Grandam cooling troubles!

This has been going on for about a year now, I have replaced the headgasket, cause it blew, had the water pump replaced , the cooling fans replaced, thermostat, fuses, everything to do with the cooling system that I can think of and do. Now, the temp will get to just before turning on the warning light, sometimes turns on the light, then finally the fans kick in and cools the car down, no heat until this happens either. Now what is my problem? i have taken it to several shops, even dealers, and they have no idea.. i have not noticed any water or radiatoer fluid under the car, there is no water in the oil.. I am about to junk this car it has been nothing but a pain.. please help

also, the car is a 3.4 v6 ( 1999 )

Have your or anyone checked your heater core? it sounds to me that if you aren't getting heat and the car is over heating something is wrong with the heater core.