Thomson (ICC17 chassis) TV: fault related to focus voltage

I have read the FAQ (and much else) without finding a clue to the
precise problem I have.

The set is a Thomson 28WT25US around 6 years old. It has a 28 inch 16:9
tube and ICC17 chassis. From about its second year there was an
intermittent fault: the set would go into protection mode (error 27: =
deflection failure more than three times). But it always used to
recover within a few minutes. Recently however it failed to recover. I
changed the LOPT/flyback transformer. That has not solved the problem
however.

If I adjust the focus voltage to give a sharp picture the set fails
quickly (within a few minutes and sometimes almost immediately) with
error 27 as before.

But if I leave the focus voltage low then the set continues to operate
for hours.

I have tried reducing the G2/A1's but that made no difference. So I
don't think it can be arcing from G2 or a breakdown in the capacitor
between G2 and ground (CB01 on the board at the back of the CRT).
Right, please?

I have read that these sets can have problems with the capacitors around
the LOPT (especially CL21 and CL22). But I am puzzled as to how that
would explain the way the set is stable with the focus voltage low -
unless (as is very possible) I am misunderstanding the way the focus
voltage is tapped from within the LOPT.

I have looked for a capacitor between focus and elsewhere which might be
breaking down but could not see one. So I am at a bit of a loss.

In short, any ideas please?

Please note I am a novice at TV repair and have only an ordinary meter -
no access to scope or EHT meter these days.