If it's jumped a tooth or two, it's also stretched or the gears are worn. Here's an easy way to confirm:
1. Remove the Distributor cap.
2. Put a ratchet and socket on the crankshaft pulley bolt.
3. Have the 2nd person standing over the distributor looking at the rotor.
4. Start to rotate the engine CW with the ratchet to the timing marks on the pulley.
5. Stop rotation at the 10* BTDC mark.
6. Again, start to rotate the engine CW.
7. Have the 2nd person tell you EXACTLY when the rotor starts to move.
8. Stop the rotation of the crankshaft.
9. Look at the timing marks on the crankshaft pulley.
10. How many degrees did the crankshaft pulley move?
11. If the crankshaft moved more than 5.5 degrees, replace the timing gears and the timing chain.
IMHO, I replace the set at 4.5 degrees of movement as a preventative measure.
The number of degrees that the crankshaft pulley moves BEFORE the camshaft transfers the movement to the distributor represents the amount of slack in the timing gears or the timing chain. This slack is seen as a time lag by the distributor and the distributor acts accordingly and the spark to the plug is now delivered at the wrong time. It's best to have the whole mechanics of the timing train nice and tight