Thanks gentlemen for your contributions. The short stroke was done about 6 months ago, with a HW seal, a titan spring and a new piston rod from Shaun at ISP. The spring guide appears to be the original metal one.
I have now stripped the rifle and lightly rubbed the seal with wet &dry, polished the piston and relubed the spring and the seals.The spring was very difficult to remove from the piston, - my new theory being that when hot, only the "free" part of the spring was working -the part in the piston having expanded and became almost stuck -hence the power loss. On cooling, the spring freed itself from the piston wall and reverted to full power. On reassembling, the power increased necessitating the removal of the spacer washer. I will now strip the rifle again and lightly rub the spring with wet & dry to give further clearance and re-chrono. With any luck this may resolve the issue.
Regards to all