Volkswagen Transporter juddering on acceleration.

my vw caddy 1.6 petrol does a similar thing, seems to be after disconnecting the battery for whatever reason, i have found that while accelerating at a particular engine rpm it occurs (eg. 15mph in 2nd or 37mph in 4th) i think something weird happens with the myserious engine control management system, as it seems to right itself after 3 or 4 days of modest driving, try taking the foot off the accelerator slightly when it happens as it is not good for the engine to the intermittently firing badly as the weight and momentum of the vehicle is transmitted back through the gears to the crank. something to do with the electronic ignition timing. may also be worth having a look at your sparkplugs and ignition cables which have to work better while accelerating.