1993 Lexus LS400 intermittent power problems

Bought a 1993 Lexus LS400 almost a year ago now, and it ran fine up until a
few months ago - One day, it started backfiring and sputtering the second I
started it up, and didn't want to go more than five miles an hour. Having just
replaced the fuel pump (and not the inline filter, ha ha, I'm an idiot), I
assumed the filter had exploded or something and replaced it. The car ran
alright sometimes after that, but almost every time I've driven it since then, it
randomly loses almost all power and can't accelerate. Even with the pedal
floored, the revs sloooowwwllly climb until it hits a certain point, and then it
immediately picks up and acts like a normal car until I let off the gas again. I
thought it was the fuel pressure regulator, but there's no gas in the oil and no
smoke out the exhaust. I got a TPS warning from the OBD reader, so I
replaced that, and no change. I'm getting more than a little frustrated trying
to find out what the issue could be.