1999 Pontiace GA Shakes In Idle

I have a 1999 pontiac grand am that started to shake/vibrate a year or so ago. It get's so bad at times that it feels like it's going to turn off but doesn't. Like I said it only happens in Drive, when I put it neutral or Park the shaking goes away significantly but it's still there but not like in Drive. Anyway, I've had to various shops and hooked up to the computer and no-one can find anything wrong with the car. It is getting very frustrating becuase no-one can find anything wrong. I've even got the dealer stumped. It's been a great car otherwise. It's a 4 cyl, 130,000 miles. Any suggestions? One mechanic told me it was the motor mounts although not broken he said that the rubber expanded therefore the engine shook. I don't know what to do anymore?????? Help!

Well did the dealer do anything about the motor mounts? Did they look at your transmission mounts? What about tire balance and wear?

Like I said it only happens when sitting idle in drive. Neutral and Park it gets a little better about 75% better and when you turn on the A/C in drive (sitting there) it get's even worse but like I said, it doesn't shut off. As soon as you start moving they shaking goes away even 1 mph. It couldn't be the tires because I'm stopped when it happens. And the dealer and various mechanics I've had it at hooked it up to all their computers and found no codes. No they did not do anything about the motor mounts because I opted not to replace them because 1) they weren't broken and 2) they couldn't guarantee me that would solve the problem. We're stumped but I would like to get it fixed. The transmission mounts are fine too according to the mechanics. Now, if it was the motor mounts (they're not broken by the way) could it be the rubber in them expanding as the one dealer told me that's causing it to vibrate and shake? or could it be possible a plugged fuel injector? But that would throw codes.

Thanks for your reply to this.

Yep, plugged fuel injector would cause a code. If the rubber has expanded enough that there is a gap in there then that could be a possible cause for the vibration. Another possibility(maybe a long shot) check your belt tensioner if there is too much play maybe your serpentine belt has a lot of play.

ok - I'll have them check the belt tensioner but the belt is tight. I had it replaced a couple months ago and when I checked it its tight but I don't see how that could cause shaking in drive and then gets better when you drive or in park and neutral. Who knows. I want to get rid of it so bad but I still have payments on it.