I had a lot of trouble this week pulling some grades with my 97 1500 Suburban (5.7L). I only had a very small trailer with an ATV on it behind me, not much weight at all. Had some similar problems last spring, pulling my boat. At that time, simply removing the gas cap seemed to solve the problem. This time, it would quickly bog down once I hit a grade, and I was only able to continue by shifting down to 2nd and keeping my foot out of the throttle. If I stepped on the gas, it would typically throw PO131 and PO151 codes (O2 circuit low voltage). I would then have to reset the codes before I could continue. A little background (I have been posting on some issues with this rig over the last year or so). In the last year, the spark plugs, spark plug wires, cap (twice), rotor, fuel filter (twice), fuel pump, fuel pump wiring harness, gas cap, and all four O2 sensors and both catalytic converters have all been replaced. I am still getting almost 65 psi fuel pressure at ON, and about 55 psi at idle. I have blown compressed air through the vent line (tank to charcoal canister) and the air is getting through okay. Any ideas on why I might still be having trouble?