Cold weather and the firing valve sticking, that's my guess.

We've had a S400 at the club today that is usually so regular it must be on Ex-lax, but it's been throwing out about a quarter of the shots at 10fpe or thereabouts compared to its normal 11.4 to 11.5. We noticed it because these shots were dropping an inch and a half low at 55 yards and spoiling otherwise good groups.

I hope you get to the bottom of this and let us all know the outcome because a Mk3 Sport is probably going to be my next rifle.

Rich