So I've done a bit more driving today, and wired the overdrive for the first time. The overdrive works fine, seems to engage and disengage without drama, but I can definitely get something to let go in low gears.
I think the clutch is okay, and it's the overdrive that's the issue. I will now present my reasoning for your consideration.
Given:
the problem presents most in first, a bit in second, almost impossible to reproduce in third, and not reproducible in fourth; andthe clutch has the most mechanical advantage in first and the least in fourth; andthe overdrive, which takes the transmission output shaft as its input, has the least mechanical advantage in first gear and the most in fourth gear
It is reasonable to assume that the issue lies with the conical clutch inside the overdrive (the annulus).
From what I've been able to dig up online, it appears the most likely issue is either the friction material on the annulus has worn, or the springs that engage it onto the output carrier (when overdrive is disengaged) have fatigued.
It may also be something simple, like the fluid level being too low. I don't expect this to be the case as I did the recommended overfill but I will check this first.
So, because this car is my daily, you once again have the pleasure of watching me throw money at it. I've ordered an overdrive rebuild kit and new springs, as well as a J-type overdrive pressure test gauge.
I contemplated ordering a remanned annulus, but it is $300 ($190 after core refund, but add shipping to the UK and I think we're back to $300). I will buy one if necessary but I'd like to inspect the old one first.
Stay tuned...