Renault Megane coupe W reg keeps shuddering!!!!?

hi hopefully someone can help me!
i bought a megane coupe monaco (w reg) 1.6, for some strange reason it keeps shuddering (and also sounds like i'm reving the car!) and the rev counter keeps slowly moving up and down between 5 and 10!!!! took it to my mechanic and he serviced the car but couldnt work out the problem, spoke to renault and they told me to bring it in but i know thats gonna cost an arm an a leg!!!!! also the electric windows at the back seem to be temperamental but most of the time they dont work!!!!!