Vauxhall Corsa jolts while accelerating?

Hi there,

While driving my car sometimes jolts while accelerating, kind of like it suddenly gets caught on something for a split second. You can hear a clunk when it happens. It only does it while accelerating, and is completely smooth when at a constant speed. The RPM do not increase over the clunk, the car just gets stuck for half a second and continues.

(Unsure if relevant but the car has a lumpy idle when air cons on and is sometimes slightly lumpy in 1st when cold although I guess thats something else)


Any idea what this is?

Thanks :)