Borg Warner v-drive problem

Hello all. I am new to this forum but hope some one here can give me some advice.
First a little history. My wife and I purchased a 1989 Wellcraft 34 Grandsport last year, and at the end of the boating season(we live in Colorado) I noticed that the port side prop shaft was spinning eratically and alot slower than the starboard side. This only happened in forward, and if I applied a little throttle to the port engine the shafts seem to match speed. Any way this winter I removed the transmission, and v-drive to have it rebuilt. There were some deep scratches in the pump in the transmission, the mechanics were not able to find anything with significant damage to cause this, but they did say the transmission had been rebuilt before, and possible was not cleaned out. Anyway they rebuilt both the trans, and v-drive, and I reinstalled it this last month.
My problem is that the port prop shaft is still spinning slower than the starboard one, although it is spinning smoothly now. The wierd thing is I checked the rpm on both engines with an aux tach, and the port engine is running faster than the starboard one.
I am at a loss, and the mech do not seem to have an answer.
Does anyonbe out there have any ideas.
Thanks in advance Phil

Welcome to ME.com

pump damage is usually caused by metal particles gouging the surfaces...if the gear was rebuilt, they should have told you to replace or flush out the oil cooler...it will collect any particles and continue to recirculate them thru the oil...

as far as looking at the shafts and assessing rpm, your eye isn't very accurate...an optical tach is the best tool...you need to verify the engine rpm and then the shaft rpm...they should be different by the gear ratio and should be consistent. If the shaft is slower than it should be, that indicates slipping within the gear (or v-drive). If slipping is occuring, further diagnosis is needed to find the cause.