Ben Feral Selinger

Hey all. I just got my new build out on the road last night for it's maiden test voyage. It's sort of a 95 runner with an 01 5.3/4l60e (ground up build, but haven't done anything to the motor yet).
Throughout all my testing before the test drive, it seemed to run fine. Idled well, had zero problems burning out the 40's in the driveway. Took it out last night, drove a few km to a buddies place, all good. But on the way home it started hesitating, sounded like intermittent misfire, big loss of power. Still idles normally.
No codes being thrown. Fuel trims all look normal between -.7 and +.8 ish. All sensors seem to be responding properly, but the o2 waveforms on both banks look super lazy. While I expect to see sharp flopping between .8 and .1, I'm instead seeing big lazy curves. That's the only diagnostic marker I'm finding thus far.
I haven't confirmed fuel pressure. It was good after the swap, and given the fuel trims aren't richening up, there's no reason to believe it'll be low FP.
My only guess at this point is that I've got bad fuel, with a bunch of water in it. Was too lazy to get a can of new gas when I was building it, so I just pumped in half a tank from the old donor truck, which had been sitting outside for a few months.
I hate guessing and testing and I've got a few hours before I can dump the tank and try again, so just wondering if anyone here knows if water in fuel would show up as a lazy O2 waveform?

Probably unrelated, but I like to throw as much info as possible, my 2-3 shift is flaring. I drove the donor truck before buying, and drove it home and it shifted fine. Now it's flaring I believe at all loads, just more noticeable when you've got your foot into it a bit. I've heard of at least a few cases where the flare was caused by a fauly ECU not letting the 2-3 shift valve snap shut. I'd be surprised if there was anything mechanically wrong with the trans as I'm pretty confident it was fine before I swapped it. Could being a bit overfull cause a shift flare? All the shifts do feel a little sloppy, but I haven't driven a stock tuned 4l60e in years so I just don't know what to expect. 2-3 shift flare RPM rises around 500rpm and takes about 1.5 seconds to complete. When it grabs, it grabs firm. Does not flare while gearing down.

Screenshot is at fairly low load. Maybe 15% throttle, steady cruise or maybe a slight acceleration. Those O2's just don't look at all right to me. I'm used to sharp peaks and troughs.