1987 300 SDL runs fine in town and at road speed but dies when I pull into rest areas or fuel stops after sustained road speed traveling. How can I increase the Idle speed and will this help the situation. Current idle speed is in the 600 rpm range and it is pretty smooth other than the above problem. The engine temp is pretty consistent at 80 - 100 C with the higher temp occurring at full throttle on hot day with AC running going uphill. Thanks in advance for any help.

1987 300 SDL
1985 380 SL
other misc domestic scrap iron