Yep...do think he has it wound up too tight. Back off the main spring tension by a full turn and see how it does. May not be real happy to be under 20 foot pounds, but mine starts acting up once I get past 18-19 foot pounds (and seems at it's happiest at about 16).
It's a surprizingly short spring travel. Bolt works a pivoting lever (left side)that works the mainspring compression, so the spring travel is NOT as long as the bolt travel..so one full turn out is a larger change than it seems.
This next one is NOT YOUR PROBLEM (probably). You tried two air tubes...the chances of the o-ring on both of them beeing bad is pretty slim.
But it's worth noting.
The other thing that can creep up on you is the o-ring that seals the rear end of the air tube. If the o-ring leaks, you won't feel it as the stock channels any leak forward along the air tube (where you are not likely to notice it).
This was a rading I got..figured I needed to stop and figure out what was wrong.
Was the o-ring on the end of the air tube. Age/hardness/crack...new o-ring put it back shooting like it was suppose to.