While setting timing the timing mark jumps back and forth whats wrong?

If you hooked the timing light up properly, it should only show the timing marks when the #1 cyclinder fires. Considering how easy it is to hook up a timing light,I feel it safe to assume that the problem is elsewhere. The only other reason for a jumping timing mark would be that there is a speed mismatch between the distributor (and the firing of the #1 plug) and the crankshaft, where the timing marks generally are. I can think of 2 possible causes. - Is the distributor being driven at the correct speed and ratio? It should be running at half the speed of the engine; 2 revolutions of the crankshaft equals 1 revolution of the distributor rotor. However either replacing the distibutor with the wrong one or (more likely) stripped gears will screw this up. - Is the rotor actually spinning at the same speed as the distributor shaft? Though most distributor rotors are keyed to the shaft, this isn't always true. My Honda stalled on the road a few times after the set-screw fell out. Friction would drive the rotor at a rapidly decreasing and increasingly mismatched speed, so the engine would run oddly for a few seconds before dying. A timing light would've made for an interesting show whenever that happened. Rolfo adds: If your distributor has a vacuum line going to it and there is a leak somewhere, like a little hole in the diaphram of the vacuum advance, or in one of the vacuum lines, that would do it. Also I have seen a sloppy fitting distributor cap vibrate back and forth causing a similar symptom.