Surging Cruise Control 83 300D

I have a 1983 300D that is in good shape. However, when using the cruise control it is not very smooth - when going over a slightly bouncy road it makes the cruise surge in and out enough to feel - like the actuator is over
compensating for the speed difference. My brother has a 1984 that does the same thing. It doesn't feel like it should be normal, but I have seen two cars do it. Is there a cause or fix for this?
Thanks.
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99 out of 100, it is the amplifier. Under the dash, next to the brake pedal.

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The amplifier has been replaced before - the original is still in there and I suppose the screws were hard to access, so the previous owner left the original one and installed another beside it (not mounted) and connected it. Does the amplifier cause that sort of problem if it is going bad?
Thanks.

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http://gdl-online.com/begin.html
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