Strange cruise control issue

[steamboatsykes]

The cruise on my 81 300D just started to behave a bit weird recently.

When I first noticed the issue (at highway speeds), it would surge, accelerate and drop out, repeating. Sometimes after setting it, it will just continue to accelerate. With that I assumed, well a bad amplifier.

But then I noticed that when I set it at speed ~50mph or below it works just fine. I suppose it still could be the amplifier, but I wonder now. Anone ever seen anything similar?