Multiple problems with 2000 Malibu

First off this is not my car it is a friend of mines but i'm working on it for her. Right now the car is kind of hard to start (have to put your foot on the gas for it to fire). After if it is started it will run ok until it is warmed up to operating temp and then things really go downhill. It will drive pretty normal until you come to a stop, you can watch the rpm go up and down slightly. If you put the car in park after this it will increase up to about 2000rpm and then the engine will kill. After you start the car again and get it to idle, the rpm jumps up and down between 400 and 1500. If you put it in gear the car will feel like it's bumping forward. My buddy has a GM diagnostics tool so we hooked that up and here are the 4 codes we pulled from the computer.
P0172- Fuel trim system rich bank 1.
P0463- Fuel level sensor circuit high voltage.
P1189- Engine oil pressure switch circuit.
U1040- loss of EBTCM serial data.

I'm at a loss for this since I don't know much about this car. If anyone can help me with this problem it'd be greatly appreciated thankyou!
Anthony

Edit: By the way it's got the 3100 in it

i had almost the same problem with my 3.1 turned out it was just the wires arcing .was a cheap way to try and fix the prob.just tuned it up and everything was alright .good luck that car is a piece.belive me im under the hood of mine almost every weekend

I would start running some tests on the car. Fuel pressure test, oil pressure test, and a vacuum test. That may give you a place to start. The ebtcm code seems a little out of place, is there an ABS light on too?