Now that I read this again, I see that I have the same issue on a 2000 OX66. When cold, it starts right up, runs at 800 rpm, then over the next 2 minutes, the revs drop. When the revs lower to 500 or 600 the engine sputters, pops and stalls. I have removed, cleaned and tested the O2 sensor and it is fine. I have replaced all filters, cleaned the VST and checked the throttle adjustments. I tested both head mounted thermo switches and checked the continuity of the temperature switch. Both t-stats were changed and checked. Low pressure fuel pumps tested OK also.
So, I am flustered but what I do now is to start the motor. After 30 seconds, I hold the binnacle in neutral but raise the revs to about 900. After 3 minutes, everything is fine. Once warm, the motor runs like a champ and starts/idles normally.
My problem is not fuel I am certain. The only thing I can think of is that the ECU thinks the motor is warm before it really is and that would be the temp sensor that I tested. I didn't test its performance curve with hot water and a thermometer though. I will wait until fall layup and do so.
If you find out what your issue is, let us know.