A simple thing to check is that the layshaft supports are in the right orientation, (these are the supports that hold the shaft for the spur gear) part no. U2449
I have a Mi1 V2 (only got it at xmas) and an original Mi.1 , I got the older one secondhand last week as car for my daughter to run at our local club.
It too was very stiff and notchy, decided to strip it all and have a look at everything, turns out one of the layshaft posts was the wrong way round, the little hole needs to be at the front on both sides, so now a smooth a silk, i was scratching my head for sometime before spotting this, worth a look.
Plus back all the screws off a bit on the top and bottom decks let it all settle and then nip it up across diagonals and then everything should find it's natural home.
