5 Ghz connection issues on Nighthawk 2100

I'm having some issues with my 5 ghz connection on certain devices on my Nighthawk 2100 router. This just started within the last few weeks. It's strange that it only seems to happen on certain devices that are connected to the 5ghz frequency. My 2.4ghz frequency has been rock solid all along (knock on wood). I've had this router maybe 2 years or a little less and don't remember updating the firmware except maybe when I installed it. I've read online that a firmware update can do more harm than good. Not sure if I might need to update the firmware now that I'm experiencing issues or try something else? 

Here is the version of firmware thats currently installed: 1.2.0.26.1.0.1

Any advice,

-Guru