Currently have cel on and the car won't go past.

Currently have cel on and the car won't go past 3000 rpm even when I am going 70 mph on the highway. Recently had the fuel system flushed because car would not go past 2500 rpm and 55 mph on the highway. NTB where I took it said that might help after reporting a P0420 code for the reason for the light. Recently have had new spark plugs installed and front wheel bearings. Oil change done and transmission fluid is good. I do hear a squealing when I first start the car that I think is the timing belt. My car is automatic but when I don't let it warm up it is unable to properly change gears and well stay at a lower gear even though the high rpms. What could be wrong and what should the next steps I take?