test drove a 98 e240 today...

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As it say i took an E240 1998 with 110,000 miles for a test drive today. looking around the car it looked great, and had a full service history. Started driving it out of the area and felt very smooth...
but...
when i got it onto the open road it drove fine unless i gave it anything more than half throttle. driving at 50mph i put my foot down, it kicked downa gear but didnt accelerate unless i released to half throttle. It also wouldnt drive above 60mph (i check the speed restristor on the cruise) then sat at a junction i put it in 1st and put my foot to the floor and it crept forward at 5 mph.. i changed it up through the gears until it was in drive... foot to the floor,, still doing 5mph... i released my foot to half throttle and it slowly started picking up speed.

When i took it back to the garage and told the guy he said he hadn't noticed (yea right) and said he would take it to the Benz dealership and have them run a diagnostic on it tomorrow morning. I really like the car/colour/spec etc... but does anyone have any ideas what this could be? could the 5-speed auto be dead after 110,000 miles?? or some sensor somewhere??
thanks in advanced