Difference In Set Speed & Actual Speed
WARNING: This page is about a different car, the 1992 Subaru SVX, 1992 Subaru Legacy, 1991 Subaru Legacy, and 1990 Subaru Legacy. However, it is still accessible from the selected car via links, so may be relevant.
Check vacuum pump and valve assembly. See VACUUM PUMP & VALVE ASSEMBLY under TESTING. If vacuum pump and valve assembly is okay, check ACTUATOR. See ACTUATOR TEST under TESTING. If actuator is okay, replace cruise control unit.