Q: Car is losing power and the temperature gauge reads high.

Hello. From what you’ve described, most likely, your engine has had a head gasket failure, perhaps because of a failed thermostat - which could cause the "temperature way up, then backs off" cycle you describe. A failed head gasket means that at least 1, and possibly 2, of your 4 cylinders are not developing normal power.

In your case, it would appear that the failed head gasket is leaking combustion pressure into the cooling system, essentially "super-heating" the coolant. The radiator may be cooling the engine down enough in some driving scenarios, but not others.

I would recommend having a overheating inspection completed by a mobile, professional mechanic, such as one from YourMechanic, to diagnose this problem before additional damage occurs, get an accurate assessment of damage and cost estimate for repairs.