Hi thank you. Rich I am giving it a rest because I have done the following and the cure may well be a angle grinder.(Ha Ha.)
Changed the following with known bits from a fully functioning 200S which is 3 years old and has not missed a beat. The problem child is a very early one? About 2002 I think.
Complete cylinder with Huma reg ( set to 11.4 ).
The barrel.
Hammer spring. Twice, new.
Probe seal. Twice, new.
Pin, with spring.
Trigger sears and springs. New.
Hammer twice. 1 New.
All seals.

So far, last year had a slight drop corrected with a new spring.
This time was running at 11.2/1 ft-lbs. with no problems.
From one pellet to the next it suddenly went to sounding 'soft'. As if something had broken.
Brought home from range and it had gone down to 8 ft-lbs. ish. Maximum. It was varying a lot.

So the conclusion at the moment is a crack or fault of some kind in the block.
Will investigate another block but I don't think it's worth the expense at the moment.

Cheers all.
Geoff.
Just an add on. Hammer clean and dry. Port adjuster screwed out and clear of port.