Why does my car loose power going uphill

This is common for inner axle shaft joints had a bad worn spot in them so as you accelerate these joints need to slide in & out freely there are binding.

I have seen many time that the rug/mat may be preventing the accelerator to be completely depressed. Check that...

A motoe mount would not let it loose power. It would however make the motor shake causing the eddect of it missing. Check the main motor mount that attaches the motor to the frame that is in front on the car just above the radiator. It has a large rubber bushing that sometimes goes bad on this vehicles. I have included a picture of what it would look like.

Suggestion only! If you had no shake before the tranny work,then retracing the steps backwards is a good way to find the new problem.
Go back to the tranny shop and complain #2 go to another tranny shop and get a professional to look at their work.

It could be a cv joint or a wore out or worn tranny mount or motor mount.

If the check engine light is on, most likely it is take it to Autozone or any auto parts store. They can scan it for free and let you know whats wrong. Or you can get a scanner like this one, its good to have. There are more expensive diagnostic code readers. These will allow you to check all sensors and then you can compare the specs with oem specs. I you plan on repairing the car your self I suggest you get a Haynes Repair Manual.

Sounds like a plugged catylic convertor.

Have you ever replaced the fuel filter? sounds like that may be the problem, it is under the car on the drivers side.

This happened to my 2001 Regal... There is a factory recall on the Catalytic Converter going bad. I had mine replaced with the recall and haven't had a problem since

most likelly it is your fuel pump is not puting godd fuel pressure
have the fuel pressure test and if it is low have the fuel pump and filter replaced