380.70 rt-n66u drops 5.8ghz

I have one computer connected to the Ethernet port of an RT-N66U, some computers on 2.4GHz and some on 5.8GHz wireless. The problem is that the router drops the 5.8GHz channel and it won't even show on the list when I mouse over the connection signal bar on windows 7 ultimate x64 after about 30...