For bouncing timing: Check the vacuum advance plate, that's what anchors the advance plate at zero. Stretched timing chain and worn gears. You may also have a worn intermediate shaft gear and or bushing. I've also found the slot in the intermediate to wide, you can pean the end of the distributor shaft to widen it for a snug fit. Last thing is distributor shaft bushings worn. You can blue print the reluctor by filing them to have the same gap on all eight. Rotor phasing is also important. You can make an extended rotor tip with brass stock from Ace hardware also. Use a quality brass cap and ditch the old one if it's aluminum. An MSD box does not cause fluctuating timing.
