Coolant loss in 1994 Chevy Cavalier

I have a '94 Chevy Cavalier with 88K miles on it. The other day I checked the radiator to find that it was low on coolant (1/3 of a gallon.) I cannot find a leak anywhere underneath the car or at any of the hoses. The heat works fine (it actually gets really hot,) and I don’t smell antifreeze from the heater. I put stop leak in the radiator and it seems to be holding it’s coolant now. However, after driving the car, I can smell antifreeze in the motor, but again don’t see any leaks. Also, my temperature guage will flucuate between 135 degrees and 175. It will go to 175, then drop all at once to 135, then go back up to 175, etc. It never goes above 175. I’m thinking that may be the sending unit, but I am not sure if the two incidents are related or not. I would really appriciate any help. I am a shadetree mechanic, but I have never run into this before.

Pressure check the cooling system with a hand pump. Trying to do the leak checks with the engine running is fraught with difficulties and hazards. If you can smell the coolant in the engine compartment, it’s likely from coolant leaking onto the exhaust.
There is a problem with the temperature gauge or wiring. It can be found by skillful use of a wiring diagram and a voltmeter.