The car is a 99 F-body. I have a built 4l60e transmission. I have a yank converter. The problem is when I leave the car in drive or 3 under wido open throttle the car launches properly and then noses over after about 60 feet, as if 6 of the cylinders went dead. If I leave the car in 1 and launch it and manually shift the car it operates perfectly. I can not pin point when the problem came about, but I do know that the only thing that has changed in the car is a volt blaster and the battery being relocated to the back. I scanned the car and found no problems. I have gone as far as to put another pcm in the car thinking that may to be to blame. Any input would be appreciated. I am at a loss. Thanks

 

Does it happen before, during or after the 1-2 shift?
Do you have any tuning (ie: shift points, shift firmness) done?

 

Before the 1-2 shift. It happens within the first 60ft of the car launching under W.O.T. The car was tuned by a local shop.