It's it great that these things are repairable? Love your work! 
It is!
I haven't really done much, beside following existing advice on this and other sites, and sharing what I have found myself.
Compiling all the data, these are the things to check:
- cc amplifier - I'm quite sure mine is in good shape now
- throttle actuator - unlikely, as it does work (as in "pulls the throttle")
- throttle linkage - unlikely, it used to stick but has been well greased since and just works
- speedometer cable - if it is stretched or worn, the needle would wobble making the speed reading inaccurate (mine wobbles slightly at ~50mph)
- speed sensor - ties in directly to the above, as the sensor reads speed off of the cable and feeds it to the CC amp
- vacuum leaks - more than likely given that my central locking works for about 5 minutes after engine shutdown, then all the vacuum is gone
The surge is by about 5mph above set speed, e.g. if I set the CC to 60mph it'll go 60-65-60 all the time.
Are there any household methods to test for vacuum leaks?