Some things to check:
Did you put the t-stat in the right way? Older t-stats are hard to reverse due to the size of the wax housing/bellows. New ones can easily be put in upside down.
Did you use a water pump adapter plate? You have to get the balancer to pulley ratio right or it may spin to fast (esp in low gears) and cavitate.
The impeller is pressed onto the shaft and it could be slipping or the 'fins' on the impeller may have deteriorated due to cavitation/age.
