Rough Idle after highway speeds- 1983 300SD

[Nuggler0]

I commute about 45 min from the office. Most of the traffic cruises at 75 - 80 at the time of day I come home. I noticed that the engine idles rough after cruising at that speed for the half hour or so I'm on the freeway. It feels like a cylinder is not firing. I need to find out how to check this out - how to check an injector to make sure it is not clogged and is working properly. Your thoughts and comments would be appreciated.