1981 Buick Electra Park Avenue

I have a 1981 buick park avenue 307 v8 engine. I have recently poured alot of money into this car and im afraid im getting robbed. Ive had a complete tune up done, new carb, rebuilt tranny, and a new muffler put on, it starts and idles perfectly, however when I accelerate it dies? Why? Ive taken the car back to the mechanic to have him retime the distributer cap but im still having the same problems? Please help.