I would contact Peter Sanford at General Development Laboratories. I
have purchased actuators and a rebuilt amplifier from Peter and he
provides excellent service after the sale and is most helpful in
figuring out what is wrong.
His spam filtering is pretty strong, so you probably want to call him
the first time and make sure he gets your e-mail on his accepted list.
I have a 1982 300 TDT that I have been driving for 16 years. It has
over 400,000 miles on it and I have had a few problems with the cruise
control over that extensive period, but it usually works perfectly.
It's rock steady and smooth at any speed and can hold speed up a steep
mountain pass when it's working right.
There are a few possible causes of surging. Your speedometer has a
sensor on it that tells the amplifier how fast you are going. If your
speedometer is jerky, the cruise won't run smoothly.
But your issue sounds like a worn circuit board in the actuator. Some
contacts run along an etched strip of conductor on the board. When
the strips of conductor become worn and pitted, the cruise will get
jerky at the speed which that part of the conductor corresponds to.
This explains why an actuator would be jerky at only a given speed
range and yet be smooth at other speeds.
Here is a link to a web page Peter of GDL was working on. I don't
think this is part of his public web site, so maybe you shouldn't
directly mention it to him because I am not sure he wants this out
generally. He sent me the link in an e-mail when we were trying to
figure out why my cruise was surging. In my case it turned out to be
a jerky speedometer cable this time. I have a pretty new actuator, so
it was unlikely that it would be a worn board.
Peter makes those boards, and I think he will sell them as parts, but
I am not sure, you will have to talk to him about that.