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iTrader: (2)

My 5.3 is cutting out in reverse. First noticed it today when trying to back up an incline. Engine revs to about 2k (3200 stall converter), drops to about 1k, climbs up to about 2K, drops to about 1K, for as long as I'm on the throttle. I initially suspected fuel starvation - tank was a little under half full. Filled the tank and tested on level ground by putting in reverse and flooring it - got pretty much the same result. Trucks seems to want to go forward just fine. From my HPTuners scan, O2 sensors are showing it to be very lean when this is happening, but injector duty cycle is ramping as expected with increase in rpm and air flow data. Truck is an 03 Avalanche with an electronic throttle body - scan is showing the throttle remaining open while the engine does these gyrations. Besides the stock fuel pump, I've got a Vortech booster pump to help feed the Vortech supercharger-fed engine. I'm thinking I'm getting a loss in fuel pressure, but don't know why it seems to just want to do this when in reverse. Backing up easy on flat isn't a problem, but when going up a bit of an incline and have to give it some gas I get this cutting out. Anyone see anything like this before?