I've had the same exact issues.
I have the same exact issue that you do. I Hacked a wii for my girlfriend and for my uncle at the same time the same way that I hacked mine but for some reason every time I try and load a game via usb on either wii's they freeze. Sometimes not at the beginning. Sometimes it loads a game and plays for a few minutes before freezing. Ive been racking my brain and searching the internet everywhere for a reason for this. This is the first time Ive come across someone with my exact issue. Please if you figure out what the issue is report back to here to help me out. Thanks