About once a week I would get only cold air coming out of the heater and the temp gauge would go to overheat but then it kicks in and I get warm heat and the gauge goes back to normal so I thought it was the thermastat sticking so I put a new one in but it didn't solve the problem. Have any ideas what it might be

 

What size engine? Could be water pump, heater core clogged, low on coolant?

 

the size of the engine is 2.5 litre

 

make sure you put the thermostat in correctly. have the EGR valve checked. check your water pump belt.