Sounds like overheating, but when game does not work in like 30 minutes, it may be two things.
1) Game disk if you play games what you get from game stores OR Flash memory if you play downloaded virtual console/wiiware games.[
2) Do you have problems with other games? If not, it can be the optical drive.
If you have warranty left, take it to nintendo to be repaired.
If you done have warranty, here is a link to an new wii dvd drive:
[product|IF207-001|Wii DVD Drive]