Engine vibrations

Good day, I live in a small rural town and I have to travel about per day to fetch children from school. My engine light has come on. I checked it with the garage and they said it was dirty air in the filters, most likely caused by the bad roads we travel on. It gives little jerks or shudders every now and then. Lately this has stopped but the car seems to lose power when travelling uphill. It reaches a speed of only about sixty kmph where it could still reach one hundred and twenty kmph. Now the engine seems to vibrate and when coming to a stop street or idling it seems to cut out altogether. I have taken it back to the garage, but they do not seem to know what is wrong with it. They say it could be a coil or the mounting but they do not know what to do. I know very little about car engines but maybe with your help we can figure it out.
thanking you.