The early Buick Electro-Cruise system does a fairly good job of maintaining speed +/- 2 mph.  However, some units tend to surge when accelerating within that range. That is to say, they don't accelerate smoothly; there's a rapid (and very noticeable) acceleration every time the car speed falls to the low end of the range.  

The Electro-Cruise power unit controls the speed by regulating the vacuum in the pneumatic chamber of the power unit, thus increasing or decreasing the pull on the throttle chain. When vehicle speed is too low, the control valve coil energizes, causing the armature to close the air port and open the vacuum port, thus increasing the vacuum in the chamber and pulling on the throttle. The higher this vacuum level, the stronger the pull, and the greater the rate of acceleration. When cruise is engaged and locked in, the coil repeatedly opens and closes the ports to maintain the proper vacuum level. If the car is at the set speed, each port is open half the time. If it's going too slow, the vacuum port is open more often than the air port. It's like two steps forward and one step back to create the desired vacuum level to accelerate the car.

We now come to the crux of the biscuit (as Zappa used to say). If the air port is partially obstructed, the result of the armature cycling will be more like 2 steps forward and 1/2 step back, due to the diminished effect of opening the air port. This will create a higher vacuum level, resulting in a more pronounced acceleration. In this case, the cure is to make sure the air port is unobstructed. You can cure this problem by overhauling the power unit as follows:

• Take the power unit off of the car.
• Separate the two halves of the clamshell by taking out the 8 screws that hold them together.
• Remove the air filter cover on the power unit and clean the filter. If you're like me, you didn't even know this filter existed until 2 weeks ago. If your car is like mine, exposing the filter will be like opening the Pharaoh’s tomb; there's nothing in there but a bunch a dust that used to be a filter. In fact, it wouldn't surprise me if the port were clogged with the dust of the former filter. I replaced my filter with the breather element off a Briggs and Stratton that I cut to size.
• Remove the control valve return spring valve and the control valve.
• Clean the air port.  You can do this with a blast of high pressure air and/or a drill bit (preferably not connected to a drill). If you choose to use air, make sure the unit is disassembled. If it isn't, you risk rupturing a very hard-to-find diaphragm.
• Repeat the reaming procedure for the vacuum port.
• Lube everything with dry silicone or equivalent. According to Mac Steiger, this alone was enough to cure his problem.
• Ed Gunyo recommends restoring and rejuvenating the diaphragm by rubbing Udder Balm into it. This sounds kind of nice. If you choose to follow Ed's advice, you may want to get your significant other involved and see what happens.
• Reassemble and reinstall.

If you don't want to remove the unit from the car, you may be able to clean the air port with the unit on the car:

• Remove the air filter.
• Ream out the air port with a drill. Don't shove it in too far, or you might bend the armature.
• Replace the air filter.