It could be a few things. Since it works sometimes it is a problem with your media.
Burning slower sometimes is worse than faster with the technology of the new burners and discs. Try burning at 4x or 8x and see if you get better results.
The other thing is your +R discs. Wiis generally prefer -R discs and for whatever reason some are extremely pickier than others. It sounds like either a picky wii drive or bad burn quality and boost up your disc burn speed