[403olds]

I have a 79 t/a with an olds 403. I have been having trouble getting a spark and starting the motor over the past few months.... the motor will crank fine but Half of the time I try to start it I get no spark.... I replaced the distributer, cap and rotor, module, plugs, coil, and wires.. its all new but I still get this problem half of the time. If i get the car started, it sounds like I have a dead cylinder until it warms up, then its fine. It also runs like that as soon as I get off the highway and will stall out if I don't pop it into neutral at a stoplight. After I realized that I couldn't figure it out myself I took it to some garages, every one says that its a different problem, says they fixed it, but after a week or so im back where I started. any suggestions?? Thanks