So i have tuned my z28 after a cam install and it runs very smooth idle and all gears except 3rd. it seems to shake the car front to back a little bit while in 3rd gear at all rpms whether low or high. my cam is 2 degrees advanced in both low and high octane,i have reduced my ltft's down so the car reads in the negs and does not go positive, i have increaced the rpm to 950. when i floor it, it runs fine in 3rd with just a little different pitch but when just cruising it is not nearly as smooth.

Any idea what causes this, what it is or how to fix??