[Doug I]

Don't know if this applies or will be any help - but it's a thought and I don't think it'll make things any worse.

This comes from my motorcycle days when I once or twice set the vlave lash - and from when I had a mini.

What I'm familar with is a small screw that you turned to set the gap then locked it with the lam nut.

Could the end of that screw be 'soft' (bad batch of them) and wearing or deforming under the loads on it. The nut would still be tight - nothing I read so far would be wearing but if the adjustment screw itself was deforming it would let the lash get bigger.

Just a thought - hope it helps.