'99 Celica, falls on its face

I own an independent repair shop and need help with a customers car. The car idles fine but when you give it anywhere from 20%-40% throttle it falls on its face. The problem is very repeatable and will even do it in neutral sometimes. Usually if you continue to full throttle it will come out of it with full power but not always. Occasionally it won't exhibit the problem at all. According to O2 readings the car is running very rich when this happens. O2 sensors seem to be responding normally. Fuel pressure is good and remains steady. TPS tests fine and I swapped it with one out of another car and the problem was still there. Map sensor tests fine. EGR system tests fine. Injector leakage test is fine. Everything on the datastream looks fine when the problem is happening except for the O2 sensors reading 0.00 or near that. I unplugged them and the car ran the same. The only thing that seems to affect the problem is unplugging the TPS. When you do that the problem is gone. However I don't think the sensor can be the problem because the TPS reading in the datastream is correct and the problem was still there when I swapped in another sensor. Its hard for me to give up and I won't throw parts at a problem hoping it will go away but I am loosing money here and am about to send it to the dealer. Anyone have any ideas. Please don't respond if you don't work on cars for a living. I don't want to wade through a bunch of try replacing XXXX posts. thanks

I'm surprised the rich readings don't result in a trouble code with the O2 sensor, but I'd suggest checking that. Unplugging the TPS causes the ECM to use a "default" air-fuel map...this may also happen if the wiring harness has been damaged or otherwise flexing such that continuity is lost and sending the ECM into default mode.

BTW, about 95% of the active forum membership don't own or run shops, yet they may just have your answer...what've you got to lose?