My cousin had a problem similar to this before he modded his wii so I don't know if the solution will be the same but, first of all he tried using a dvd lens cleaner (he is a smoker and was told that the smoke could leave deposits on the lens?????).
After cleaning it it worked a bit better but not perfect, so he took it to a friend who opened it up and cleaned the inside of the wii especially the lens with ethanol (almost pure alchohol). He has had no problems since.
It might be worth trying a lens cleaner first and probably not bad practice for good maintenance anyway!!
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