My '79 116 has the OEM vacuum diaphragm and amplifier setup for the cruise control. The amplifier and speed sensors are rebuilt / new. However, the cruise continues to lose several MPH on a ~ 20 mile highway drive. The speed just drifts down over the course of the 20 miles, prompting me to reset the speed higher.

If this is a vacuum problem, why would the amplifier not detect this lower speed and correct the problem? Or, as perhaps somewhere could explain, is this a symptom of a maladjusted amplifier?

Thanks, -H