Help - Engine, Tranny or Brakes

Ok guys - My wife has a 97 Vert with the 6 in it. The car has been parked for a while, but we just started driving it again. It has an Auto and around 60k miles.

HEre is the symptom - Occasionally, when the car is in drive, but I am stopped at a light or in the driveway, the idle goes eratic, and you can really feel the car straining in the brake pedal. If you shift out to neutral or park, the idle smooths back out. I really tend to notice this when the car is on an incline, like my driveway.

Any Thoughts?

Well, after starting to drive the car again, I changed the plugs, ran some injector cleaner and a tank of premium through it and did an oil change.

Would a misfire present itself like this? Not sure why it would smooth out after shifting back into neutral. Wouldn't it continue to miss?

Thanks for any thoughts!