1996 Oldsmobile Cutlass 3.1 L no heat. Temp guage fluctuate

Radiator was flushed new thermostat installed after temp guage started going from normal to hot and back again etc. Problem persists No anit freeze leak in crank case or external or exhaust. One mechanic told me head gaskets, another mechanic is not sure, pressure tested slow bleed down, asked if it was original water pump. It is. Car is 1 one owner. Can you help? Thank you.