Revs going silly??

[KirkJames]

Right, had my car into Fast Fit this morning as they hadn't done the center section properly and was rattling like a good 'un. Took about 20 mins, they put a new center section on and I drove away. Stopped at the lights and now the car is going from 1200 revs down to about 400 then back and so on and so on. Any ideas why this should all of a sudden start happening and how can I stop it? The car is slowly starting to upset me with all the little niggles it seems to develop.