1998 Deville loaping in gear.

I have a 98 deville that idles smooth but when put in gear, (either forward or reverse) it surges or loaps. I see very little fluctuation on the tach. There are no error codes. It's more noticeable when the car is moving slowly. Could this be a transmission problem? Fluid is OK. It runs fine on the highway.

Update: finally got an error code. P0300, Engine Missfire Detected. It didn't narrow it down to any particular cylinder. Where or how does the computer detect a missfire.? Put new plugs in, no help. Should I start changing parts, (plug wires, coils)? Where is the timeing sensor on this engine?