The cruise control on my 1980 450SL tends to lose speed the longer it is on cruise control. I can set it at 70 for example, and gradually it will decrease the speed at which it is trying to keep. Also, it sometimes surges and decreases while trying to sustain a speed, even when on flat ground. Any ideas what may cause this?

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Chris Meyer
1980 450 SL (99k miles)
2003 Honda Odyssey (Family Hauler)
2006 BMW 330i (daily driver)