N only and N/G modes have the problem, G only doesn't, it sounds to me like you have a cipher type/encryption mismatch going on. Check the sticky at the top of the forum.
The settings you need to look at are WPA / WPA2 , AES/TKIP/Both , but I can't recall what you are going to need to set to, and frankly it may vary. Also, set your 20/40 to both/auto whatever it is.
good luck and please post back your results