Cruise Control Accelerates But Won't Hold Speed

At around 55 mph or above I can move my cruise lever to set/accelerate and it will immediately start to make the car accelerate. As soon as you let go
of the lever, the acceleration immediately stops, but it won't hold the current speed.
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blanket resolder the amp and replace the crappy old dried out capacitiors with modern low esr, rock steady, pin point accuracy can then be expected.
this is what my '79 6.9 was doing.

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I was heading that direction... thanks.

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By the way, without me digging out the worthless Chiltons and Haynes manuals, where might I find the cruise amp?

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the reason I suggest a blanket resolder is because 90% of the time they die, its because of bad soldering, so you mayaswell do it, while its open.
take your time, and do it properly, if you are daunted by how much work there is to do, don't do it all in one sitting, as you'll be really unhappy if it starts to not work at all.

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The car is an '81 300TD Turbo.
Don't mind the soldering. Well versed at it (fix numerous electronic devices).
Thanks for the input so far!

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Opened up the amp and nothing looks out of place. Nothing overheated. Was this the case with your's also?

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