I can't vouch for what's sold now.
My springs have proven themselves by lasting all these years on nothing less than .600 lift. What else can I say other than I'll run them as long as the pressures stay up.

1970s and counting

As far a motor not revving...I agree with you guys. It can be gas delivery(pressure or adjustments), timing, valvetrain and possibly internal engine.

I'm a big fan of common sense..I would never jump to the conclusion it's springs before eliminating other potential problems first