3.3 Flex Fuel Won't Idle
 

2000 Grand Caravan
3.3 Flex Fuel

The engine runs fine for a while then hesitates. Once it hesitates, the engine will not idle unless you keep the throttle open a bit to keep the RPM's up around 1000. It does this intermittantly so it is hard to find the problem. The only code that has shown up was P0172 -- O2 sensor system too rich bank 1. It only showed once and cleared itself.

I have replaced the idle air control motor.

Any help would be appreciated. thanks.