The good neighbor policy on "N" mode will cause the wireless to do that if it thinks it hears another WiFi signal(maybe even a microwave oven?), it will default back to a 20Mhz channel instead of 40Mhz without you doing a thing, then you think the router is bad but it is programmed to do that(it is written in to the 802.11N standard)....... Being the drop is from 130 to 65, I would put money on it. If you don't want the good neighbor policy, go back to a draft "N" firmware.
Tested my speed last night, 75 x 3(seen peaks over 100), 9ms ping time. I refuse to pay for the "big boy" package. Guess I will just have to live with it! During peak times I slow down to 35 X 3(The Horror!).