[Grim Reaper]

I agree with Cruiser. Probably timing chain. The quick check is turn the motor clock wise till you get to TDC. Take the distributor cap off and then turn the motor backwards till you see the rotor turn. Look at your timing tab and see how many degrees it turned before the rotor turned. 5 and under is probably just al ittle slip in the worm drive at the distributor. over that its starting to sound like the chain is loose and as the lifters crest on the timing lobes it will jerk around and give an eratic timing.

Check the mechanical advance as well under the rotor. Somethimes the weights and inner shaft will get gummed up and cause simular problems. If the weights feel slugish at all and doesnt snap back it's gummed up.

To clean it pull the distributor. Remove the weights. Drive the roll pin out of the gear. Slide the inner shaft out and clean it. Use a good high temp THIN LIGHT grease. I have been using white lithum grease and it seems to work well in this application. Reassemble and check it.