Very pleased you've now been able to identify the source of the issue David. Definitely not a guru BTW, geek maybe
who loves delving into what info is out there, given I have a vested interest as owner myself. FWIW it was a pure shot in the dark that I suggested looking at cam solenoids in the first place, but the wiring looked vulnerable.
Unfortunately I have no idea which cam solenoid is likely to be the cause of issue. From an electrical point although the resistance of each measures the same, one could give a path to earth, but probably only when energised, hence reason the fuse doesn't blow immediately. Nevertheless you could measure for resistance to earth/chassis on each. I would expect they are isolated from same though, so as with your thoughts, replacing connections one by one until culprit is found may be next path. Good news is once you have identified it, replacements seem to be in the order of £30 or so.
The fact that running is now improved suggests that maybe one is/was sticking.
Reaching out to Mustang forums or dealers to see if this is an issue is another suggestion.
ETA: yes Bressingham is a popular local venue for me. Particularly enjoy the steam model weekend beginning of May although not this year
