> Any and all thoughts on this are welcome. It is mounted on the steering
> column (2 screws) and the cable then goes into the steering column cover.

In addition to the control lever, there is a control box in adjacent to 
the engine compartment.

> Above 55 mph it surges, up and down, and then cuts out. Sometimes it just 
> cuts out! But it will not hold at all?

With some luck this problem is related to the ones below, and could
be fixed easily. However a cruise control system is a feedback
control system and with the wrong unit surging and (in control
theory lingo) instability is quite possible with a properly functioning
unit if it is not matched to the vehicle/engine in question.

> When it is working, if you
> 1) blow the horn
> 2) put on the turn signal
> 3) dim or raise the headlights
> and probably other things I haven't found yet! It cuts out - as though you
> had tapped the brake.

My first guess for this problem is that it is related to voltage
fluctuations being interpreted as "off" signals. You can try:

a/ Check voltage and ground connections. If you have a voltmeter
check so that voltage is constant during case 1-3 above.

b/ Check all other wiring. 

c/ If a and b doesn't help try moving it's positive voltage connection
to another fuse. Try both a circuit separate from 1-3, and the same
circuit as the brake light.

Do you know from the prev. owner if it ever worked right? The off/
cut out signal could have been wired wrong.

If all else fails find a EE control major and trade some help with
his homework for some cruise control debugging. (noticed you're at
umass. Nice school!)

Good luck,
Martin