You say "it seems like". Do you mean to say it is intermitent, or did it start cutting out and then finally fail? If that is the case I would look first to replacing the relay in contention in this thread.
http://www.fixya.com/support/t224708-amp_cuts_out_after_while

It's a cheap fix and will eliminate one potential cause. The Radio Shack part is widely available.
http://www.radioshack.com/product/index.jsp?productId=2062483

My RS-300 had started dropping the left channel, but occasionally both. It is possible that the relay has failed on one side and thus you have a dead channel...I did the fix and am now quite happy with the amp

Steve