Need assistance 96 Geo

I have a 1996 Geo Tracker 5 speed 4X4 that idels rough. It only does it when the car is warm. Sometimes my car will die cause the RPM's drop so low. I have a DTC of P0135 which is the heater circuit on the O2 sensor #1. I've come up with nothing. I didnt want to spend the 70 dollars for a new sensor and find out it was something else. Any help will be appreciated.