I have a 1977 Spider 1800 that will suddenly begin to buck when driving down the road. It doesn't seem to matter what gear but RPM is around 3000. I have to push in the clutch and let the engine idle and then carefully limp the car home keeping the RPM down. Any suggestions? The car starts fine and is tuned by ear. It idles at 850 RPM quite smoothly. I haven't verified this but believe the timing is at about 15 degrees advanced on plug # 1. The distributor has a Pertronix Ignitor installed in place of the points.