2003 Chevrolet Silverado Low Fuel Pressure

My Silverado set a CEL the other morning, pulled code, was for Knock Sensor #2. But when I went to pull it in garage it cut off, would not restart, just crank. So checked Fuel Pressure, only 40psi. So I replaced 2 Knock Sensors, harness, and fuel pump module(dealer parts). Drove it to work next day, was perfect, ran better all around. Set off on trip, went 300 miles, cutoff when stopped for gas, no restart just crank. Towed home, pulled Intermittent Crank Sensor Code, and rechecked fuel press for fun, KOEO 50Psi, KOER 47psi, unhooked regulator 52psi. So I grabbed a CKP sensor and fuel regulator. No more code, running great, still same fuel pressure, why? Fuel Trim -.8 to 3.5% short or long. Looks good on scanner runs great, im stumped.