My '81 300SD idles poorly, especially after a long highway run. It starts easily, but within 20 seconds, it starts to shake untill it warms up. Then, it stumbles a little. Sometimes it stalls. If I rest my foot on the accel pedal to bring the idle up from 800 to 950rpm, it smoothes out somewhat. It has 304K miles, burns less than a qt of oil every 3000 miles, makes good power, accelerates ok. But, if I take it on the highway for an hour or more, it shakes violently when I return to idle. The body needs work, so it isn't worth much. I'm hoping there's a fairly inexpensive fix. Could it be injectors? If it turns out to be only one injector, can I get away with replacing it, or is it recommended to get a whole set? - Ric