1998 Buick Lesabre stalls with acceleration

I replaced the engine in my 1998 Buick Lesabre
about 4,000 miles ago (crate engine with new
spark plugs, thermostat, water pump, intake
manifold and distributor cap) and about 2 weeks
ago, after I got some gas I started to stall when I
accelerated after a stop and my engine seemed to
be missing while accelerating uphill. I began to run
Beer through the fuel system believing it to be
water in the gas. 4 tanks of fuel later (I drive a lot),
an oil change, and lots of frustration, no
improvement and a code for my upstream O2
sensor. Replaced the sensor and had mild
improvement, but still experiencing missing and
stalling. Considering changing the fuel filter again.
Also experiencing rough shifting when the rpm
unexpectedly drops. Spark plugs are tight, battery
is charged, turns over and idles perfect. Also
thinking of looking at the catalytic converter. Any
suggestions?