classic symptoms of air in cooling system.
couple examples of how it can happen (both happened to me)
91 conti.. 3.8v6.. heater core has a pin hole... coolant drops over course of a week or 3 to a level where I get no heat, and temp starts to fluctuate. I add coolant (it was low, checked it in radiator, not overflow bottle, do it when engine is cold), typically less than 2 quarts of coolant low... usually didn't take a full 2 liter bottle of mixed antifreeze. there was a VERY faint antifreeze odor in car... barely noticable!
92 cougar, 3.8 v6.... head gasket starts to go... (note, these are essentially same engine, just one setup for fwd, other rwd, and if you have a 3.8 in your 94, its the same deal).. it would leak a tiny bit of coolant into one cylinder while engine was cold, once warmed up it would seal. Gasket finally failed 2 days before I was going to change it. When I would check coolant (again, in rad, not bottle, and cold) it would be low, but the system would be under pressure!!!! blowby and leakage thru gasket from combustion... it also had no heat and fluctuating temp gauge...
Find out for certain if your low on coolant. I don't trust the owners manual's statements that checking in the overflow bottle is all you need to do on our cars (note: some have a pressurized bottle, those you need to check there), but ours is not under pressure, its just a catch can. I have seen so many radiator caps allow antifreeze out when they get hot and pressurized, but not allow it to suck any back in when it cools down again.
The absolute last thing you want to do is overheat a 3.8.... all sorts of bad things can happen after that.
Again, verify that its full of antifreeze... I keep my 91 filled to bottom of filler neck on rad. The cougar is different, the top of rad is below the high point on engine, its sometimes a bitch to get the air out of the system on it!