Renault Espace, ecelaration shudders???

Ive just brought a 2002 Renault espace automatic, petrol, 89000 miles.

superb condition inside and out. however when i am driveing occassionally when ecelarateing (mainly between 30 - 40mph) the car begins to shudder and volt.
it seems to be happening more regulay the longer it drives for. would this be the ecelarator cables or gearbox when changing gears???