Q: Temperature gauge is fluctuating, sometimes showing overheating.

Hey there. There are combustion gas analyzers that you can buy and use on a DIY basis for this, but I would have a professional perform the diagnostic, and hopefully find the fault elsewhere (other than head gasket leak). You’ll have to check out the entire cooling system before you get to the head gasket. The radiator could be plugged, could have plugged cooling passageways, could have a defective water pump, loose belt issue, collapsed hose, excessive load on the engine due to dragging brakes, underinflated tires, and so forth. Also, if the engine timing is off, the engine running too lean, or the exhaust is plugged (catalytic converter) those conditions can cause the engine to overheat. Quite a number of possibilities have to be looked at. If you want to rule out the worst, certainly you could check for exhaust in the coolant on a DIY basis. For help getting this checked by a professional, I recommend having a certified technician from YourMechanic inspect the overheating concern so that this can be corrected.