I am having the same issue. I have a red wii with 4.3u.
I have downgraded to 4.1U and then upgraded back to 4.3u. Still wiimote will not sync on both versions.
Only way I can sync wiimote is to start Wii, under health screen, press and hold sync button by SD card for 15 seconds. Press sync on wiimote and then press sync button near SDcard.
The wiimote stays connected while on the system menu and in homebrew channel.
I lose sync if I start a game, open a channel and restart Wii.
Once I restart the Wii, I have to unsync wiimotes by holding sync button 15 seconds again and go through the process.
Any one have any ideas how to fix this?