What are the steps to properly adjust the cruise control actuator cable? I've got the old-style (vacuum). Does the cable need to be tight or have a specific slack to it?

For a short time, I got it to hold 70mph steady yesterday. The rest of the time it picks a speed (55, 60, 65, or 70), powers up to it at WOT, drops to 0 throttle at the set speed, then 5mph below that set speed it goes back to WOT until the set speed is reached again. It will do this until I hit the brakes/clutch/off. Hitting resume will do the same. Decel.set won't do anything. Accel.set will only make it run WOT until the stalk is released.

Any thoughts?