Loss of Power

Engine cranks, starts and runs perfect during warm-up period to include idling and acceleration (approx 7-Miles) then "like someone flipped a switch" the engine begins to misfire and lose power down to "limp-in mode". The code P0171, right engine bank lean, appears. Immediately upon cool-down, engine starts and again runs perfect until warm then goes through the same misfire episode again. I have replaced all sensors, the distributor w/coil, plug wires, plugs and EGR valve system but the symptoms have remained constant. The problem began when the engine would crank but not start. Replacement of the Crankshaft Sensor solved this problem but now this new one has been created. Spark Plugs show extremely white tips indicating a lean mixture but upon removal of fuel line, there appears to be pressure and fuel. Replace fuel pump and fuel filter. Not showing any codes but same symptoms Stumped