2.4GHz channel gives me speed up to 300Mbps, then drops to 1Mbps

With my Centrino N-2230, I can connect up to 300Mbps. However, once I hit this max speed, a minute or so later, my speeds drop to 1 to 5 Mbps. It also appears to screw up all of the traffic on the 2.4GHz channel. The 5GHz channel remains unaffected.

If I set my R6300 to "Up to 217 Mbps", this doesn't happen, or if I restrict my N-2230 card to 20 MHz.

I tried to get help from Netgear, but they have been of little assistance.