This is mostly a note to myself...
My printer has about 43 days' run time now. That sounds like a lot, but I've had it almost 5 years. So it's only been running 2.4% of the time.
Anyway, it occasionally messes up a print by having the X carriage lose position. Usually, the X carriage goes up too far/doesn't come down. I don't know if the carriage has ever gone too low, usually too high. Occasionally, like maybe three times in a month. Or less.
I've tightened the belts. I've checked the motor wiring. I've torqued on the pulley set screws. Problem still persists.
This morning I searched the Interwebs for new ideas. I found this very helpful advice from KAS
Turn the printer on and Home it, that should lock the steppers. See if you can grab the carriage and slide it down the tower. If it's loose it should allow the pulley to slip while the shaft is locked by torque.
So I tried it. It was easy to see that (a) the X carriage moves easier than Y or Z, (b) the motor pulley turns when the carriage moves, (c) the movement has the zip/step feeling of a stepper motor rotating.
So if the motor is rotating, that rules out the belt and pulley. So I suspect a bad connection. I removed the motor cable. I pulled the Molex connector apart AGAIN and checked all the crimps AGAIN. (They are all good.) But the connector housing
is pretty worn, and the pins weren't staying in place. Fortunately, I had a spare housing, so I replaced it.
Same symptom. So I wiggled the connector on the RAMBo. Symptom gone. (X carriage has as much resistance as the others.)
I think the Molex SL connectors that RAMBo uses are not so good for high vibration long term use.
So this problem is still open.