Nissan Micra M reg

Hey, i wonder if anyone can help me - i know nothing about cars!! I have a nissan micra M reg (1996 i think) which has done approx 80,000 miles. The car has been in my family since new and weve never had this problem

When driving with foot on accelerator the car suddenly jilts forward and back. Its as if im throwing it into a low gear or slamming my foot on the brake. When my foot is on the accelerator it bounces back and forward like its got kangaroo petrol in it - very annoying. It doesnt seem to have much power to it during this time. Its not all the time - like it was fine from the garage for about 10 minutes and when they were driving it they said it was fine.

I put it in for a service (£140 - which is prob more than the value of the car) there and they said they couldnt find any problems just cleaned out gunge from the fuel tank and replaced a few bits and bobs but still has this problem!

Please Help.