Hi Paul,
I'd check the TV/Kickdown cable is functioning OK. I'm guessing if its stuck this might result in something like you described?
Also does it get serviced regularly? Check the transmission fluid/filter.
Also if he changes the filter himself, its worth adjusting the bands whilst you are in there. You'll need an inch/pound torque wrench.
Hope this helps as they are all easy/cheap things to try, otherwise it could get expensive
Daniel