The same thing was happening to me previously but on the 2011 rerelease and it was annoying the heck out of me but I fixed the problem on my end.
Make sure you put a dab of thread lock on PE1 (page 8). The gear housing needs to latch completely in place in a way that you shouldn't see a seam in between the top and bottom housing when sandwiched and screwed together. The "lip" of the top gear housing should overlap the gearbox plate and not just sit on top.
****Update****Oops, sorry. I just realized that the Q was on the 2001 and not the 1988 model so my comment may not apply as I am not sure if the 2001 mirrors the same gear housing.
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