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Car "running" then sluggish (95 grand prix)

I’m currently driving a 1995 Pontiac Grand Prix with 170k miles on it.

I recently got an oil change with a fuel injector “Gumout” service (I know, wallet emptying scheme). After leaving the place I drove 5 miles home, then 50 miles on the freeway followed by another 100 miles on the freeway. I had no problems over this trip and the car seemed to perform quite well (although my gas mileage dropped to under 20 mpg which is normally my minimum for a tank of in-town driving).

After the freeway driving I had another 70 miles of highway driving. During this I had 2 problems. As I approached a stop sign my car wouldn’t drop under 2000 rpms and kept pushing to accelerate despite my foot not being on the gas and braking (idle is 1000 rpms or lower normally). I shifted into neutral and the engine started revving up even more so I accelerated through the intersection and kept going. The next stop sign the car slowed normally and seemed fine til I tried to accelerate. I had no power at all and I rolled forward slowly. After stopping again I waited a second and I was able to accelerate normally.

My main concern is the transmission (which may be wrong). I have noticed the scent of coolant (I had the water pump replaced 4 months ago) which seems unusual but also seems unconnected with the problems I’ve had. I should note that the temp was between 8 and 20 degrees Fahrenheit outside but the car was running at its normal running temp when I had the problems.

Thanks for any help.