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Hi, there are only a couple of possibilities for this problem. Either the idle valve is letting too much air into the motor or it is getting in somewhere else. Block off the idle valve and see if it stalls. If it does then you know that the problem is too much air bypassing the idle valve. If that is the case you need to replace the idle control unit. You might also need to replace the valve but start by replacing the control unit. The O2 sensor has nothing to do with the problem. Also make sure that the ignition timing is set correctly.
Let me know if you have any questions.