Up & down internet speeds

I recently installed a Netgear AC1750 router (which was purchased from Cox store) and while the speeds are higher than they were before (sometimes up to 110mbps) it often slows to a crawl (< 1mbps).   The only way to get around this is that I have to switch from the 2.4Ghz band to the 5Ghz or vice versa.  It gets awfully annoying having to constantly switch from one band to another....any ideas on why the wide range/inconsistency of speeds?