3800 V6

I just bout a 93 Buick Lesabare w/ a 3800 Series I V6. When the engine is cold and your driving, it lugs. But only when its shifted into overdrive, if you speed up with out making the transmission downshift out of over drive the engine lugs like its in 100th gear. It also does this if your going up a hill or and load is put on the engine with out the transmission shifting out of OD. Now if you run it in 3rd gear the engine does not do this. It only does this when its cold, one the engine gets warm, the problem goes away, anyone have a clue? By the way, I thought it might have been the lock up torque converter solenoid but a mechanic ruled that out for me. Maybe the crankshaft sensor? Comments welcome.