When I am going down the highway, my A/F guage says it is "Stoich". After a few minutes of driving my car will feel like it has a miss and then the guage goes all the way lean. If you give it gas it will go back to rich or stoich but will eventually buck again and go lean. After a few seconds then it will throw an O2 code on the CEL. I have replaced the O2 sensor since it seems the logical thing to do but it does the same thing, this is why Im thinking the TPS, since it is at a certain position on the throttle when this happens. I have checked the voltage to the TPS and it is fine. The fuel pressure is at 38 psi with vacuum and I have tried lowering and increasing the fuel pressure and get the same thing. any ideas? anybody had a TPS die and have similar symptoms? I also have a Field SFC that is hooked up but not turned on, could this possibly have something to do with it?
This is a ZC in a 90 Hatchback SI.

Thanks
Gary