You can do a quick check on the tension for the advance by pulling the distributor cap and twisting the rotor. If you do it slow enough you will feel two different spring tensions, and it definitely should not flap back and forth other than about an 8th of an inch. Cams don't walk like this to alter timing, cam walk is front and back, which is why I mentioned the roller cam and button, lifters are set on the center and side of the lobes to keep them in place, so it would have to be a loose timing chain to make the timing jump around, but you alleviated that issue, so the only other timing item is the helical gear, which can move up and down, but not likely except how I talked about, so it has to be the distributor advance itself. Springs, which are about half an inch long, can break, so that's where to look for a problem.