Are you using a MSD or similar control box? If you are, then the timing will jump some depending on which pulse your timing light picks up. The way to check it is to dial your timing light up to your total timing and then check your timing when the engine is revved up to the all in point or just a little above that rpm (maybe 3500 or so). If it is dead on then the timing light is picking up different pulses and all is fine.
Jack