Chevrolet Repair: over heating, bars stop leak, headgaskets

Question
2000 chevrolet venture with a v 6 motor about 177654 miles on it it heats up put a new radiator on it about 6 months ago cause it was leaking and befor it started runging hot the temp gauge would move up and down it would go half way then go on up.the heater core had a small leak so thry put a half of bottle of bars stop leak in it.could that have stoped it upcause the day they put it in it ran for about 3 days befor it started to run hot.or could it be the thermostat causeing it to run hot.oh yes the heater will put out good heat for a few mins and then it will get cold then in a few mins it will get hot again....i am trying to just help this older lady get it fixed cause she dont have any money to take it to the shop   thank you and god bless

Answer
GOOD MORNING CHUCK,THIS COULD BE A COUPLE THINGS. I WOULD LIKE TO SEE IF YOU COULD GET THIS PRESSURE TESTED TO MAKE SURE THERE ARE [[NO EXTERNAL LEAKS.]] IF THERE ARE, FIX THIS FIRST.  MAKE  SURE THE COOLING FANS COME ON WHEN SITTING IDLE AFTER IT WARMS UP. THIS IS VERY IMPORTANT. IF NO LEAKS ARE FOUND AND THE FANS COME ON,, THE NEXT STEP WOULD BE TO PUT A NEW THERMOSTAT IN AND HOPE FOR THE BEST. WE HAVE HAD SOME ISSUES WITH BAD HEADGASKETS ON HESE ENGINES..BUT LETS NOT JUMP TO CONCLUSIONS JUST YET..START OUT WITH BABIE STEPS AND GO ON FROM THERE..IF ALL OF THE ABOVE IS ALL GOOD,THEN YOU WILL NEED TO ASSUME THAT THIS ENGINE HAS BAD HEADGASKETS. THERE ARE TEST FOR THIS BEFORE YOU RIP THE UPPER HALF OF THE ENG APART..LET ME KNOW JOW YOU MAKE OUT TO THE FIRST 3 STEPS AND GET BACK TO ME IF YOU WANT TO KNOW HOW TO TEST FOR A BAD HEADGASKET...GOOD LUCK TO YOU AND I HOPE THIS HELPS YOU..TOM...