Hey bert
your freinds x10 must have been built incorrectly (maybe to much end float in the prop,,or motormount not being sufficiently tight)
the x10 has been tried and definatly tested and dosent have any transmission issues as long as the pinion saver rings (supplied in kit) are fitted
as you probably know my little Bro now owns ttech and if there was a real problem we would now be talking about it honestly and getting to bottom of it and putting it right
at my local club the pred outnumbers all other makes considerably and they all run fine
as burgie rightly said (if built correctly virtualy boombproof)
cheers