I think you may have answered your own question, we had very similar problems only a couple of weeks ago along with the wifi struggling to reconnect after we came back into range.
I split the bands and all was fine again, in fact I have purposely not connected to the 5Ghz network as it is more important for me to have the greater range than the increased speed (I live in the middle of nowhere so no issues with the 2.4Ghz being crowded)
Paul.