I've been noticing that my 83 300SD is starting to speed up and slowdown when the cruise is on. When I go uphill, everything is fine. But when I am on flat ground the car surges constantly, and if I put my foot lightly on the accelerator I can feel the cruise actuator really pumping. I mean the accelerator probably moves up and down probably faster than once every second.

Is this the cruise control amp starting to burn out?

Mine is doing this too. It would be nice if I could fix it.

mine did that for a while, then it just died all together. where is the amp located?

The amp is located right under the steering column almost agains the firewall. Its a silver aluminum box about the size of a flask.