02 Grand Prix Idle Surge Rough Running on acceleration

I have a 2002 Grand Prix SE 3.1 Litre V6 with 123,500 miles - it has been running rough lately and I have noticed considerable vibration/shaking on acceleration - thought it may be a motor mount so had a buddy visibly look at it with me - it did not look like the motor moved more than normal to us but another symptom emerged - with a steady foot on the gas pedal at 4000 RPM - the car acted as if I was revving it - there was an idle surge up to 4200 then a drop to 3800 again with steady pressure on the gas - I have the tires rotated and balanced every 3000 with oil changes so don't attribute the vibration to tires or balance - not sure where to start - the car has never been tuned up - do I need a tune up, fuel injectors, computer? I would appreciate your comments - don't think it's the transmission as the shift is smooth and I don't hear any clunking or odd noises - Thanks for the help and I did make a small donation - appreciate the forum!