Mercruiser 350, Alpha one outdrive (late 1980's). Just had the motor/engine aligned and outdrive installed. After launching at idle going to the slip I could feel a thump in the driveline every once and a while (30 seconds or so), as if you hit a log underwater. Does it at any speed. No engine skip. When NOT in gear, it does not make the thump at any tilt level or steering pitch. No abnormal noise in the driveline (thru hull exhuast, so listening with ear to hull at idle not in gear) When underway at higher speeds you can see a "bump" in the prop wake when the driveline binds or slips. I will be pulling it out of the water hopefully this weekend after completely adjusting the shift cables (it was shifting fine, but its worth a try). Possible causes: U-joint, gimble bearing, bad lower? The drive was supposedly rebuilt..... If anyone has had a similar problem, or needs more info please let me know. Would like to get an idea before diving in.