Cruise Control
 

89 300 TE cruise control will not hold steady speed, very slight variation 2 to 4 mph, enough to make the ride uncomfortable, would this be a vacuum leak??
Where do I start?

Could be the electronic control module. On 123s, 126s, and 107s it is often necessary to open it up and resoldier points on the circuit board. I'm not sure how it is configured on a 124. Nevertheless, you should read through this thread and the ones linked within it.

http://www.peachparts.com/shopforum/showthread.php?t=227190&highlight=resoldier