Do you use any devices that need A and B modes? Usually most current devices don't need B mode so I've experienced this and usually tell others to turn off those modes that are not needed. No need to broadcast modes that you don't use.
2.4Ghz is more reliable since it's not impacted by things like walls and doors and building materials as much as 5Ghz is due to it's high frequency. 5ghz is best used in line of sight or at most, one wall in between. 2.4Ghz should be great on single mode G and 5Ghz should be better on single mode N.
Any cordless house phones? yes but not near to AP>
How far away and what frequencies do the run?Any other WiFi routers in the area? yes>
How many? I recommend using
InSSIDer to see whos near by and what frequencies and channels they are using.