2001 pontiac montana. bought it 3 wks ago. i think i bought a lemon! help

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first week heat went out. we flushed the heater core. heat wrked for 4 days. heat went out and engine over heated.im not leaking coolant or oil and they are not mixing in the moter. and i replace the radiator cap. replaced the water pump and thermostat. called the (ma&pa) dealer. no suggestions on what could have happened or whats wrong, didnt wanna buy it back, excuses on why he cant trade. he said he had the van for like 4 months. so it could have been a push pull drag it in. we think they may have had problems with the head gasket. i poured bars leak for heead gaskets and colling systems (black cap). i know thats one of the worst things to do. but i need to buy my self some time to come upon 1000 for parts and repairs to replace the head gasket. i also noticed that there may even b a problem with the raidator stem. any suggestions. no one will really answer my questions. can i get through 3 weeks with the bars leak? can they flush the bars leak out when they do the gasket?