First off, I’m not yet clear on your symptoms. When the bike “loses drive” does the back wheel drive at all – even on the center stand? I am asking because there are two clutch packs, and one possibility is that both clutch packs are engaged at the same time, or that the single clutch pack (that should be fully engaged) is slipping.
Two gears selected at the same time would point to the selector valve, the pressure bleed valve on a clutch pack, and maybe the high pressure bypass valve sticking occasionally (although this doesn’t seem that likely).
A slipping clutch pack could be the selector valve, the pump, the bypass valve sticking open, the oil screen partially clogged, a worn clutch pack, bad clutch pack o-ring, the wrong oil, and more I can’t think of right now.
I would check the side stand adjustment – assuming that it may be moving the selector valve when it should not move. The nice thing about this check is that it is easy and costs nothing but time.
Otherwise, you could always drop the pan and check the oil pump inlet screen – just to make sure it is clean. If you wanted to be more invasive, you could also drop the oil passage body – to make sure it is clean.
You could also check the transmission line/oil pressure to eliminate pressure (too much, or too little) as a potential cause – especially if you could check it when the symptoms occur.
I know there’s more to check – these are just the quicker, easier things – before you get into the pump, the selector valve itself, etc.
I hope others at their two cents – so we call all help you out!
