GM-GMC: poor acceleration: hesitation, lean condition, schrader valve

Question
I have a 97 GMC K2500 5.7 liter vortec with 130000 miles and recently the truck  loses power under moderate to heavy acceleration (power significantly drops right off). If the throttle is eased on slowly you feel only a slight lag but their is a general lack of power. I cahnged the fuel filter but the problem persists. The fuel pressure at the schrader valve is 40-50 psi. The service light came on recently and showed a faulty O2 sensor before the catalytic converter. Would the O2 sensor create this problem or do you think the pump is on its way out? Thanks


Answer
The fuel pressure seems low to me.  With low fuel pressure often comes false O2 sensor codes, usually alerting to a lean condition.  Check for a restriction in the fuel system, or, consider the posiblity that the fuel pump is failing.
hope this helps.