Ordered new upstream O2 sensor, Denso part number 234-4162. Checked for vacuum leaks, using carburetor cleaner. Cleaned MAF Sensor with MAF sensor cleaner last week. (wires renewed with OEM, about four years, 50,000 miles ago) Old plugs looked good. $52 234-4162 Oxygen Sensor with 4-Wire 10.5” Harness for Non-California, Non-4WD Lexus/Toyota 234-4162 
Renewed downstream sensor with new two weeks ago with Denso part number. Swapped coil-pack feeding Cyl's 2 & 5 to another cylinder pair (no change. Goes from Open Loop to Closed Loop at what appears to be normal operating. Other tests are Green (see screenshot from Torque, below) Prior to code P0136 and performance issues, was getting flashing CEL and codes P0300 (random misfire) and codes P0302 and P0305 (misfire cyl's 2 & 5), above 70 MPH, but only above 70 MPH, but no noticeable missing or performance issues. Previously had code P0136 (O2 Sensor, Bank #1, Sensor #2). Idles rough after warmup, stumbling (I've been shifting to "Neutral" at stop-lights, to avoid stalling). Mileage has fallen to about 10 MPG, City. Power during acceleration seems follows a "step pattern". When cold and accelerating from stopped, VERY Sluggish. Acceleration improves after full warm up and as RPM's increase, but still not normal. Very low power until it warms up, especially at low RPM's. I have to go through about three cycles, before it will start, and then it stumble-starts Don't want to buy another, unless I have some level of confidence that is the problem. 
I suspect catalytic converter, but that's just a suspicion. I'm hoping to get some guidance/suggestions on rectifying my problem. I am experiencing a performance problem with my '99 5VZ-FE 3.4L V6, Automatic Transmission, 4WD Taco with about 235k miles.
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