N 7260 2.4GHz channel width / connection "speed"

I'm trying to understand why my two laptops, that both have 2x2 N wifi cards, will initially connect at 300 speed (40GHz width), then drop to 144 speed (20GHz width) within only a few seconds. I don't have any BG devices at all and I have confirmed the two laptops are the only two devices on the 2.4GHz ssid. They are both sitting in the same room, 15 feet from the router, no walls between. One laptop is brand new Lenovo, the other is Toshiba, both have Intel 7260 N cards. I have a brand new router TP Link C8.