Help with 1990 ECCS Diagnostic

[aerovin]

My 1990 Maxima suddenly started running extremely rough with little power and will barely move. When in park at high rpms it runs a bit better. The ECCS Mode 3 code indicates a bad exhaust gas sensor but, when replaced with a new Bosche unit, had no effect. Spending a bit of time with the ECCS unit reveals the following: the car runs poorly for about 1 minute, with neither red nor green LEDs on the ECCS; after 1 minute, it runs okay for twenty seconds, with MODE 1 green LED steady and no red LED and MODE 2, both red and green LED on steady; after twenty seconds, it goes back to running rough with no LEDs illuminated in either MODE 1 or MODE 2; after about 25-30 seconds, MODE 1 shows a steady red LED (corresponding with the check engine light) and MODE 2 shows no LEDs illuminated. The MODE 3 code that comes out is 33 (exhaust gas sensor). Electric ciruit for the sensor seems fine and, again, replacement with a new sensor had no effect (bad new sensor?) When it is running poorly, the exhaust smells like it is running rich. I am stymied and looking for help.