2007 Opel Corsa back firing

I'm 72 and the agents are telling me that It might cost me about R 5.500 to fix my car. About three weeks ago I got stucked as it wouldn't start, I waited a bit then it started, but while I was driving it made a sudden noice as a gun shot but it kept going. Then the other day, it just cut out but after a short rest, it starts again, then it goes for a distance and suddenly it cuts out again. Needless to say, I just wait a bit then start again and off I go. It has been OK for the last three or four weeks with no problems, untill last Friday, when it decided to play up again, same story, waited and restarted and she goes!

What causes that? The car is serviced on regular intervals at the dealership, and the spark plugs were replaced in 2011 but they say it may be cables or plugs or coil - I don't know what they're talking about, and they seem not too sure as to what it can be and I worry that as usual, they like to just replace parts than to troubleshoot and find out exactly what the real problem might be, and I can't afford to soend that kind of money. Coud you helpor suggest what to do?

Best regards

Horst