My engine has been stumbling only on cold starts. It feels like the engine is missing for about 3 to 5 seconds after startup until I rev the engine a few times makes it go away. It only happens on cold starts. If I dont rev the engine the problem lasts longer. One time the car would not start until after several minutes of cranking and resting even with holding the throttle. Weather: 20-43f Radiator pressure test: lost .75 psi at 12.5 psi over 31 minutes. Brand new timing belt, radiator, hoses, water pump, oxygen sensor. System was just flushed with irontite. 24,500 miles on fuel filter. Tapped on idle air control while it was stumbling but it did not help. It runs fine after the initial stumble. Any ideas?