Volkswagen Repair: Oxygen sensor/fuel pump, mass air sensor, mass air meter

Question
I own an '01 jetta 2.0L.  When I press on the gas the engine accelerates but then immediately decreases in power.  Sounds like it's about to stall but doesn't.  It's sounds very close to stalling but NEVER does.  If I let the engine heat up to normal engine temp.(190 degrees) the car runs ok for a brief time period(few minutes).  Then the temp drops like a rock instantly to zero and it lacks in power and sounds like it's going to stall, but again, it never does.  I replaced the o2 sensor with no improvement(front o2 sensor,not rear on the exhaust manifold). Could this be a lack fuel going to the engine or is the computer messing with the sensors thus tricking the car into thinking there is something wrong when there really isn't.  

Answer
If the temp drops to  zero after the warmed up then the engine sending unit ECT is bad. Its in the coolant flange drivers side of the engine. The loose of power sounds like a bad mass air meter, throttle body and rarely the coolant temp sensor ECT. Check these. When cold unplug the mass air sensor and see if the problem still is present. Doing this will turn on the engine light and store a fault. if PROBLEM IS STILL PRESENT THEN LOOK ELSE WHERE.