I have a mazda 323 w reg uk. For around the last year the vehicle has been shuddering when started but never when the car is started on the first drive of the day, it always does it after you have driven it on a short drive parked it up and then go back to drive it around 10 mins later. The exhaust shakes and looks to be kicking out more fumes than you would expect the car shudders and the revs drop down to under 1000 revs, the vehicle struggles even with the foot down but then eventually it drives out and it kicks back to normal with a small smell of burning . I have had the spark plus changed tried putting cleaner though it but still no change but it does not do it all the time very frustrating. I've had it into a mechanic but like all these things it never done it whilst he had the car. Any advice would be grateful some people say it could be an air filter or maybe the exhaust? It's due a service and m.o.t and i would like it resolved without being taken for a ride by the garage if you know what i mean