The simplest explanation might be that the Make/Break switch on the R/Hold relay is gapped too close so that it closes with vibrations during gameplay and fires the Game Over relay.
Jet Spin Game Over relay (resized).jpg
/Mark

In addition to what Mark says, check the slam switch on the coin door to make sure it is adjusted properly. This is a switch with a weight on the end. It needs to be solidly closed at all times. If it's barely making contact, then during game play, the vibration of the flippers or other coils can make the switch open, and that will release the R (hold) relay, which will drop you immediately into game over.

There are almost always similar slam switches on the bottom board and in the head (usually located near the replay unit) that will cause the same problem if they are out of adjustment.

- TimMe

The slam switch seems a little more likely. One reason there are are multiple slam switches is that they're each sensitive to motion in different axes. The one on the coin door is sensitive to front and back motion (like a kick to the coin door). The one on the bottom board is sensitive to up and down motion (like lifting and dropping the game). Another if present might be sensitive to side to side motion.

And also the hold relay R itself. These coils tend to get fried and weaken. Three coils I always replace in these 70's games, 1st ball, Game Over and Hold.

If yours has a brown crispy wrapper (R) time to replace.

@pinhead52, have you seen these coils weaken or do you just replace them as a precaution? I can understand how a coil might open (and not work at all) or short (and overheat) due to long term heat exposure but I'm unsure how a coil might weaken.

yes when these coils do get crispy they can become flaky and drop out during game play. as a matter of pratice i replace if the wrapper is falling off