Hope this helps anyone with a similar problem...
Had a daunting intermittent issue where the idle on my (late 1996 build) 60 Series would "stutter" from the normal 700 rpm up to 1000 rpm the normal high idle speed. I would not characterize this as a galloping idle, which is a different issue that I also experienced in a different life. With the current problem I would also experience a periodic uncommanded high idle for several minutes (which became interesting at stop lights or backing into a parking space). A local Detroit service center focussed on the engine EEC by swapping in a new EEC to see if the problem would go away. Of course, the intermittent problem never occurred during this trouble-shooting effort. The high idle system is apparently quite complex, passing through a number of inter-related components and circuitry.
Since I had a trip planned for Nashville last month, I scheduled service with Prevost in Goodlettesville, TN to work this issue. Fortunately, as they were moving the coach into the shop, the gremlin struck. After a laptop analysis and some brainstorming by the old-timer's they discovered the fast idle relay (R22) had a bad ground. They cut wire 953 from terminal 87A and wired it directly to a chassis ground stud. Problem FIXED!
Kudos to the guys at Prevost.