I think that the problem is in the actuator. I have found that the actuator can have worn spots and will surge or have worn traces on the feedback and will give you fits. The actuator causes more problems than the amp and it is mostly the cause of the amp going bad except for the rare cases of cold solder joints in the amp.

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1977 300D Lost coolant while someone else was driving
1983 300D Can't run without oil
1985 300SD (gone but not forgotten)
1990 300TE 4matic Sold
1991 Yamaha Venture
1975 Kawsaki 250 triple
1974 Honda 200CL
1951 8N Ford
2008 Wildfire 650C