My 79Z had the engine rebuilt 10 years ago and has about 25K on the motor. I blew the head gasket on it 8 years ago and had it fixed but it never ran the same since. I was getting on the freeway and accelerated hard and that was all she wrote when the head gasket went. Yesterday i passed a car and as soon as i got passed and let off the gas some it started to surge bad. The rpm's will go up and down and the car will pickup speed and then slow down with light acceleration. It idles fine and will accelerate fine as long as i give it enough gas. If i am cruising and hold the speed steedy it will surge until i give it more gas. On deceleration it will surge bad also and free revs just fine. Any ideas and what to check first? It feels like someone has a rubber band on the bumper and it will pickup power and then pulled back 1/2 secong later.