More on rough idle/shaking

I tried 89 octane gas (owners manual says to use 87), and my car seemed to idle smoother (when stopped, in drive), but it idled unusually high and the engine shook when it was in park. Does this help narrow down at all what the problem with the rough idle/shaking could be? Does using a higher octane gas than you are supposed to need cause more carbon buildup?
(car is a '94 GA GT 3.1 V6, 64,000-something miles)
Thanks

could be of the mix of diff ocs of gas, but thats just my guess... I could maybe understand if you've used a lower oct for most of the time you've had the car and then switching to a higher oct .. stuff like carbon build up and what not could play a factor.. but this isnt an educated guess :)

You could have a misfire here or a bad vacuum leak, if you are not familiar with tracking this down, have someone qualified look at it.