Manual transmission, I assume. Otherwise the post makes no sense.

If the transmission tail is moving while you're driving, it could easily cause it to jump out of gear.

If the transmission tail isn't moving, but not sitting in the right place, it would likely affect the linkage adjustment, possibly causing incomplete engagement and allowing it to jump to neutral.

Check that first. But don't let it jump out of gear. At all! That does serious damage to the transmission, which will be very expensive to fix. If you can't rely on it staying in gear, don't drive the car. Consider it broken, or soon enough, it really will be.