2005 Volkswagen Polo Juddering when accelerating

Hi, I wonder if anyone can help me. My polo (2005) has started to judder/vibrate when accelerating, which is particularly noticeable at low speeds. It seems to be coming from below the gas pedal but it doesn't happen all the time and I usually have to drive a few 100 metres before it starts. When I take my foot off the pedal the juddering stops.

I took it too my local garage and although they could tell that something wasn't right they couldn't find where the problem was coming from. They eventually replaced one of the driveshafts but it doesn't make any difference. I've since taken it to the VW dealer for diagnostics but they couldn't even detect any fault at all which is really unhelpful as something is clearly wrong!

Both garages tell me the car is safe to drive but how can they be sure if they don't know what the problem is? I don't enjoy driving the car the way it is and I'm worried about breaking down somewhere inconvenient.

Has anyone got any idea what the problem might be? Many thanks for any suggestions!