Your comment about the serpentine belt system gives me pause.
V-belt drives have the crank and water pump (all of the pulleys, actually) turning the same direction. As a general rule, serpentine belt systems will have the crank and water pump turn opposite directions to get more wrap on the crank due to the added force on the belt.
If you didn't change the water pump for the serpentine drive, that could be your problem right there.
