Unwanted downshifts over bumps

[Duncan Smith]

Gears on the MTB (Shimano XT) are all set up nice, instant changes, no
noise - perfect on the road. But when pedalling over bumps (pot
holes, off road etc) the chain-ring deraillieur has a tendancy to down-
shift by one towards the inner-most sprocket.

This is confimed in the trigger pod window as it will either have
moved from 3 to 2, or from 2 to 1. To make sure I wasn't knocking the
trigger I fisted the left handlebar grip - still does it.

Any thoughts on where I should be looking to solve the problem - in
the pod or at the mech??

Many thanks,

Duncan