Marauder FT decline in accuracy

My .177 Marauder Field and Target has been in a funk for the last year or so. Velocity used to be pretty consistent, usually within 5 fps between shots; that has increased to 15 or more. Shot groups have become inconsistent as well. I suspected it might be the regulator so I replaced the Crosman one with a Huma with really no improvement. I have cleaned and polished the barrel, changed the barrel o-ring, changed the air stripper o-ring (shroud was loose over air stripper), tightened and loosened the shroud with no improvement. I also changed scopes with no improvement. I did notice that after I cleaned the hammer and tube that the hammer became difficult to cock. There is wear (shiny areas) on the top front of the hammer and the bottom rear. I lubed the hammer with a light coating of moly paste and it got much easier to cock but was still inconsistent. I've got over 16000 shots with the rifle so could wear be a factor? Thanks for any advice.
 
If accuracy is decreasing along with velocity variation, don't forget that changes in the pellets themselves can do that. We often just assume that new tins are like old tins, but that is not always the case . . .

As for wear on the gun, I had a Marauder that I shot probably around 20K rounds thru and the valve stem had worn the valve block increasing clearance. This led to increased variation in speeds and thus a degradation in accuracy. I replaced the valve and was back in business.