I have a mac pro 3.1 year 2008. I did an upgrade to 20gb ram and GPU changed to HD 5770. recently the memory rear fan is not working, ending up the beast shuts down itself due to the heat. I replaced it with a third party fan, same size and width. Everything is assembled accordingly, however it only spins during boot and it stops after that. Front fans and PSU fan are working perfectly fine, just the memory rear fan is not! what is the problem?