This may help to shed a little light on the situation.
I have the same problem with my 93 Toyota PU. It started after I returned from home for Christmas to find that someone had cut and stolen my catalytic converter. In the process, they took the oxygen sensor and left the wires hanging.
The strange thing is I do not have a California Toyota but there are two sets of wires hanging. one has the harness which I believe is the one to the O2 sensor and the other has the harness ripped off.
I ran a straight pipe for the time being until I can afford to buy a new catalytic. It ran fine at first, then started hesitating and would rage once I hit about 3000 RPM.
I thought it may be bad gas beings it started right after I filled up an almost empty tank. I put a bottle of fuel treatment in her and she ran like a beauty again, for a little while. Then it started to hesitate again. I ran it like that for a while and added treatment one more time. While the treatment may help a little, it still hesitates. So I got to thinking, when I filled it up that first time, I also ran it through the car wash. As someone mentioned above...theirs started after going through water.
I am currently waiting for all of the rain to stop so that I can get into a junk yard and have a look to see what the wires go to and maybe pick up the proper harness, O2 brackets, and O2 sensor.
Hope my story helps...any ideas?