With the Custom Cruise III, about all you can do (without changing modules) is check the basics of the control systems for the cause:
1. Inspect the vacuum system (hoses and components for vacuum leaks).
2. On the electrical side, check for loose/broken/oxidized terminal connections.
3. Be sure the linkage (between the servo and throttle) has no excessive play and is correctly aligned. Procedure as follows:
1985---87
Servo Unit Adjustment
To adjust servo on V6 and V8 gasoline engines, close throttle, then with ignition and fast idle off, adjust rod assembly length to give .020---.040 (Exc. C, K, R and V models with V6 engine) or .007---.015 (C, K, R and V models with V6 engine) inch clearance between rod and stud.
A couple of basic vacuum system dwgs:
Electrical schematic:
This is GM's diagnostic procedure for Custom Cruise III with high/low set point problems:
Cruise Speed Above or Below Set Speed
1.Perform preliminary checks.
2.Perform electrical and vacuum system checks. If no system malfunction is noted, replace module and recheck system operation.
And this is all they offer regarding adjustment of set point:
Engagement-Cruising Speed Zeroing
There is no provision for engagement-cruising speed zeroing. If cruising speed is too high or low, check vacuum hoses for improper routing, restrictions or leaks and servo linkage for proper adjustment. If cruising speed remains too high or low, replace the electronic controller.
I know that isn't what you wanted to hear, but there is no magic bullet.