Nintendo wii help please!?

My friend bought a pre-owned wii today and then we tried playing a game earlier but the cursor kept playing up, appearing on screen and then disappearing, i recommended we move the sensor Bar bit lower and it worked for a little while but then did the same thing, he then borrowed his neighbors sensor bar and its working perfectly just not on the Mii channel. Also the main controller doesn't have a normal back panel, it has a battery pack one, what could be the problem, the sensor bar? The controller? or the console?