Can you solve the problem below for me?

My Y Reg Renault Megane starts to shudder when we are driving along, and then loses power, and we have to get into the side of the road an shut off engine and wait a minute and then start up again. This has happened 3 times and Garage has replace all 4 coils twice, but is still happening. It is dangerous if we are in the passing lane and loose power. The Garage does not seem to know what problem is. Please can you help.