Q: Car overheating

I started having issues with the heat in my car a month ago the temp gauge would go to hot then back to normal then to hot when it was idle it would go to hot also when the gauge was at hot the heat would blow cold air I wemt to VIP they said waterpump I got a 2nd opinion and was told thermostat I had it changed they checked my head gasket said the seals were good and did say I had some corosion on my water pump with some seepage. The thermostat being changed made my car go back to normal 3 days after it was fixed I had a 2 hour car ride on the way back the temp gauge starting going to hot and then back down (same symptoms) I wemt to VIP they said my coolant was a gallon and a half low they filled it and said they thought I had an air bubble from changing my thermostat it hasn't over heateheated sense putting coolant in my oil looks normal no white smoke coming from exhaust . What's going on??

My car has 142000 miles.
My car has an automatic transmission.