Likely overheating just like a PC or 360 will turn off to prevent damage. Check then vents, if there is lots of dust use compressed air or a vacuum to remove it. That is assuming the cables are snug and the powersave settings are not activated.
This is especially true if you're a smoker or have pets.
XBL: WiiVault Wii: PM me PSN: WiiVault
PC: AMD Athlon II Quadcore 635 (OC to 4.0ghz) , ATI Radeon 5770 1GB (x2)
MacBook Pro C2D 2.8ghz, 9600m GT 512 iMac: C2D 2.0, X2600XT 256