Your problem is that you try so use the 645 as if it was a 35mm camera. AF on medium format is for the patient and subjects that, preferably, don't move. If you are trying to do focus tracking or whatever, just focus manually and get over it. If you really do need AF then shoot the 35mm and sell the Mamiya. Simple as that.
As for those you say are shooting weddings with a Contax 645, I've read interviews of a few of them and they all said they focus manually.
So don't expect that dropping a few $$$$ on a Contax is going to solve your problem.