I had a similar problem. I could get the duty cycle set at idle but when on the freeway it would move out if range and vice versa. It turned out to be the fuel distributor. I cleaned it with brake cleaner solvent and the problem went away. Since your adjustment keeps moving I would consider the possibility of something associated with the fuel delivery system.

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I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k