Shuddering issues! 2001 Mercury Sable

Our car went from working perfect to slowly - over a good 10k miles just having fits!
Going up hill if you accelerate the car shudders and won't speed up until you let off the gas and even still it continues to slow down w/ no shudder. It also shudders within minutes of the AC being on.
Also - there is a burnt smell when we come to stops. Wondering if that is part of the same issue or a brake issue? But I checked the brakes and they were fine.
We have been given a couple of diagnosis - from a catalitic converter - b/c the code came up for O2 sensors (we replaced them) to another code that came up recently about the torque converter.

would really like your opinion.