Have the exact same symptoms. Holds fine at lower speeds, disengages if set at speeds above 55. Right now I have two circuit boards for Cruise Control in my hands, checking components, traces, solder joints, etc. (visually only, but under magnification). On the board in question, only item I have noticed is that a BZX diode has a bit of heat discoloration at the soldered leads. Don't know if this is due to previous repair or if it has significance. The other board holds any speed just fine, but will not resume if the brake ground has disengaged it. It will resume if disengaged at the stalk switch. Electronics is not my forte. Will report any findings, even if I just send one of them off to GDL and try the resolder technique on the other.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)