2003 200 HP S/S spitting fuel at warm idle

Hi all,
I have a 2003 200 HP Saltwater Series with OX66 that is giving me fits. The engine starts up fine as usual and idles at 800 until warm, as soon as it lowers down to 600 it slowly begins to fall back to 400 or even 300 rpm's, it begins to spit from one side of the exhaust port what i believe to be unburnt fuel. As it spits the fuel it slowly falls back and stalls. Restart is at 3 or 400 and then stalls out again very shortly after. I have already changed the water separator and both diaphram fuel pumps with no results. I have'nt been able to check the plugs yet.. will do so in the AM. Fuel is new gas from auto gas station, not marine. Could this be a fuel injector stuck in the open position? Any and all assistance would be greatly appreciated.
Chet

The O2 sensor is clean as a whistle. The engine has only 35 hours and has been used sparingly in the last 3 years (work sucks). The engine also runs fine while above 1000rpms (smooth while in gear and idle with no unburnt fuel). The problem only occurs at the low rpm range 600 and lower. I will be checking the plugs in the morning to see if any are wet after stalling. Sorry for not knowing, but what is a TPS?
Thank you very much for the response.
Chet