Distributor seems to jump out of timing?

MoparMike1974 said

Good point. If it were jumping time there it would be either 180° out or correct.
Pull that distributor out and check for wear. Maybe the advance plate is floating around loose.
Another good point...some intermediate shafts have the gear pinned to it. If that pin is sheared it will appear intact until you give it a rev and it will slip. Again, never seen it happen on a SB but its a very common problem on /6 with the nylon gear.

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Mopar small blocks don't have a gear on the distributor shaft, just a slot in the pump drive and a corresponding tip on the distributor shaft.
The pin coming out wouldn't allow the distributor to jump time unless the shaft was jumping up so far as to slam the heck out of the contact button in the cap.

#1, check the reluctor to pickup gap .008 or so is fine.
Chk the collar and pin for up and down play and the cap for rotor button damage.
Looks ok?
Pull the reluctor rotor off and advance plate to check for the springs on the weights, and that the weights move easily and return.
All good?
Could be a bad pickup coil in the distributor or ignition box problem.

One more bit of info is that big block and small block distributors are different in rotation so they advance differently but look the same other than the pickup coil rotor position on the shaft and the direction the vacuum advance pulls.
This means if he happens to have a big block distributor in a smallblock the rotor phasing would be way the heck off but the engine may run some if the plug wires are installed in the smallblock orientation. (clockwise firing order)