Chevy Cavalier engine trouble?

[brainiac5]

My car is a 97 Chevy Cavalier, automatic. I recently had the serpentine belt, spark plugs, oil filter, etc. replaced, about 3000 miles ago. Yesterday it started acting up. The engine seems to rock intermittently when it first starts up, and when shifting between the lower gears, there seems to be a power loss, and occasionally, I can hear/feel a knock under my feet in the center of the vehicle. What's the prognosis?

[stephen c]

You would not think it would be fine for 3000 miles then act up if someone replaced the spark plugs incorrectly. It should have acted up immediately. However it is possible something not replaced is now bad. I would do a simple fuel injector/throttle body cleaning first before they try to sell you on a new set of injectors. Could also be a clogged or restricted exhaust.

[derrickisonline]

Who did your tune up (spark plugs). It could be misfiring because they didn't do an appropriate job or didn't replace the distributor cap, rotor, and spark plug wires. If they did they may have hooked them up incorrectly.

[kelly_f_1999]

tune up and bad gas id try a fuel treatment first most likely you have some water in the gas every common or it needs new plugs