of course it could have been a dodgy bearing on the turbo but i would have expected this to be spotted before fitting an im sure you would have checked for play in the shaft and spun it with your fingers and would have noticed the rough feel of the bearing, only other thing i can think is that the turbo was not primed properly or you have an oil feed problem if all seemed ok on first inspection and the previous turbo did the same thing, im not taking sides here mate just tryin to help so you dont have similar problems in the future