my mercedes s500 96 seems to start and run but idles very very rough like its miss fireing even more so when i put it into drive or a gear like its about to conk out it did it last time a few mounths back but i put it into a lower gear to get it home then it sprang into life and ran fine since now i left it for a while its done the same i dont want to go out and have it srping back into life on a roundabout again what can i do any ideas out there