1993 Oldsmobile Bravada Car does not start properly

1993 Oldsmobile Bravada mileage: 133,000. About twenty percent of the time the car will not start properly. It may start but very rough for three to five seconds, or not start at all until cranked over for about ten to fifteen seconds, when it finally catches. Once running it is fine. I had it to the dealers, where they did a complete tune up, plugs, wire, distributor cap, rotor, fuel filter, and several sensors, which did not help at all. Then they replaced the fuel pump, and dampener, which did not help. It makes no difference whether the car is parked uphill, downhill, or level, or how much gas in tank? It makes no difference whether the car is warm or cold. Sometimes it starts fine, and other times it doesn't. Any helpful suggestions. The dealer’s methods of replacing parts is getting expensive, with no results. Thanks!