2002 GP has no power

I have a 2002 GP that just has no pickup. It idles fine. I replaced a battery and have had nothing but troubles since. It threw a TPS code and when I looked I found the TPS was unpluged. How this happened I have no idea. I plugged it back in and still made no difference. The shop replaced it all together and cleared all codes. Now their telling me that I have 2 cylinders that are not firing. I find this hard to beleive. There are no codes being thrown now. What happens is when you drive at low speed the car seems to move and run ok. When you step down on it and require power it just flattens out and won't go. You might be able to do 20 miles an hours. Could this be a Cat or O2 sensor without throwing a code. I would image if it was it should kick a code. Any ideas?

have you done the usual...check fluids, replace plugs/wires/filters etc... all can be done for less than a $100, having any power issues with accessories?