Dont know if this is any help you may have already elminated reg creep.
Some time ago i had a simular problem I was shooting FT at Lea Valley and my first shoot in each lane went low, second shoot bang on.
So went home and tried it over the chrono and found that the first shot after the rifle had stood for a few minute gave a lower reading and then all shoots were fine over the chrono, leave it again and the first shoot was low. A reg service sorted it out.
As i said may be of help.