Glad they got you pointed in the right direction. As soon as I read it, I was going to say hub also. I think it's the Mercury props (Tempest, Trophy, Fury) that have the flo-torque hubs, which are the easy ones to fix. I think the Yamaha props are pressed in hubs and can be a bit of a pain. That is why I run the Mercury props, plus they just run better on my boat.
I use to blow through hubs like crazy. As soon as I hit 75, it would take about 45 seconds and it would let go every time. After going through a bunch of hubs, I finally talked to a mechanic friend of mine, and he said the lower unit on my boat has a passage that needed to be drilled out just a little wider to allow for more water to flow through the hub and keep it a few degrees cooler. He widened the hole for me, and no more problems. But, I've also managed to do a number on my prop, so I haven't seen 75 in about a year, so I'm just going on his word it's fixed...haha