Unwanted downshifts over bumps

Since 2001, over 90,000 cyclist's have joined Cycling Forums to discuss topics from general cycling to equipment, training, racing and travel or vacation destinations (especially in europe during the tour de france). We also feature an great deals in our online store, 100's of articles, classifieds and product reviews.

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