Many folks, including me have had the CC "slow down-speed up" disease. Many, many threads on this forum with same symptoms. Almost all have fixed the problem by replacing control unit box with a known good one (I did) or sending their unit off to one of the repair facilities. I appreciate your electronic "chops" (I was an electronic tech in a former life) but it sounds like replacing the obvious bad caps etc aren't going to fix it.
As far as the servo, I've never seen anyone find that your particular problem was caused by the servo! Some folks have had good luck with this guy: (no affiliation)
Porsche 911 928 Cruise Control Computer 92861712700 | eBay
For $79 I think you will put an end to your frustration
Good luck,
Grant