Hi
It is possible that you have the transmission stuck in 2 gears at once, this can happen if the selector forks are worn. the gear lever inside the box can jump the selectors leaving the main box in a gear but the leaver appears to be in the neutral position therefore when you select a gear the box is in 2 gears at once. I suggest that you remove the top cover assembly from the top of the box & ensure that the selectors are all in the neutral position in the box (this will obviously entail removing all the tinwork around the top of the selector housing). This may be a pain but is easier than removing the transmission assembly.
I have personal experience of this both on 101's in military service and in a 110 of my own after an Ashcroft recon!
You may be lucky and find that a new (unworn) gear leaver will improve the situation.