1984 300TD cruise control problem...

Hi, all,
The cruise control engages but it doesn't hold the speed. It fluctuates constantly from 2-3 mph below to 2-3 mph above the set speed in a cycle that
it is about 10 seconds long, guaranteed to drive you out of your mind in less than a minute.
I am trying to figure out if I should look at the actuator under the hood first, or the control module under the dash. Of course, there could be some other cause that has escaped my scrutinous eye...
This has all the feel of a fairly common (or not unusual), straightforward problem with a fairly straightforward solution. I just don't know what it is and have had little success finding it on my own.
Thanks much in advance for any help or advice.
Ken
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Couple possibilities from my eyes...
First, does your speedometer jump around a bit during any driving conditions? For example, my speedo used to be jumpy when accelerating through 40mph from a stop... my cruise control would jump around as well no matter the speed I set it at. Culprit was the speedo cable, shot some WD40 down it and it works like a champ and the cruise is fixed. Might also need a replacement.
Second, it could be your actuator. Inability to hold speed sounds like it needs a rebuild...
Hopefully its the former.
-Mike

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see: www.gdl-online.com
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