We have a 2002 Saturn L200 with 4 cyl

We have a 2002 Saturn L200 with 4 cyl. 2.2 motor. About a month ago we had problems with it accelerating and after believing the transmission was slipping and having it checked at a transmission shop found out the cat. Converter was plugged. We had that changed. It does run better but since then we are having problems with the way it runs. In drive at a light or stop sign it shakes back and forth and acts like it is going to die once it gets below 1000 rpm. The radiator fan runs from starting the engine to stopping it instead of kicking on and off like it should. The car comes off cold on the gauge on the dash almost as soon as you start it instead of taking a bit and it is now getting 14 mpg instead of the normal 22. We used 2 cans of sea-foam, one of which at a half tank to make it stronger and we also replaced the plugs which were fouled with carbon that built up with the converter plugged. We are also getting a lot of carbon black out of the tailpipe. I know this is a lot to take in but it makes no sense. I have talked to a couple of mechanics and car people about it but they are all stumped at what to try next. Any suggestions? Thanks!