Heat then No heat?

Good morning Aveo forum, new participant here. Just bought my first 2010 Aveo sedan Lt. 84500 miles on it. Bought it from a co-worker who recently had issues with the overflow tank cap, it suddenly blew off the tank while driving down the road. He refilled and replaced the cap. He also has had the head gasket replaced and head resurfaced along with replacing the timing belt from a previous incident. I drive 43 miles to work, morning till afternoon. While driving with the heater on, i get cold air then hot air. I have bled the system with cap off. I also have hooked my diagnostic scanner to it while driving to view live data. The coolant temps will range from 185 to 240 constantly, back and forth. This is why, im guesing, as to the fluctuating heat issue. T-stat opens then closes. Also, while parked and running, it will stay at a constant 203 to 206. To my knowledge the t-stat has not been replaced and has the plastic base still. Do you recommend i replace the t-stat? If so should i go with the updated metal base? Any help appreciated and glad to be a member. Thanks for reading.

Not the tstat, replaced and still have same issue. Found resevoir cap loose again while doing driving diagnostics. Gonna replace that next. On a side note i took both heater core hoses apart from the heater core and only received flow while raising the rpms to around 3500. I think there should be some flow even at idle if pump is working correctly. Yet sitting at idle is when the temps run the lowest.

Can someone tell me what the normal operating temps are for this sedan lt? The new tstat opens at 221 degrees.

Not sure what the temps are supposed to be, but I wonder if the heater core is partially clogged, since your getting a heat/no heat situation. Is the coolant
clean? I'm wondering if the head gasket is leaking again.

Its also possible the water pump isn't pumping properly and needs the extra RPMs to really get things moving.

Or, if the water pump has a plastic impeller, it may be melted (happened on my jetta).

Sorry guys have been busy hunting lately. As it turns out, the mechanic that did the head gasket replacement the first time took it back and re-did the head gasket. This fixed the issue. I now have a 2010 sedan LT with new head gasket(twice), a new timing belt, new water pump, thermostat, temp sensor, reservoir, upper radiator hose and oil cooler gasket. All with only 85,000 miles. Not too bad, lol. Thanks for the replies.