Gigablast speed keeps dropping to 25-30MB`

Hello - about 6-8 months ago I signed up for Gigablast.  It worked great.  My speed wasn't ever close to a gigabyte, typically between 200-400mb.  In the past week it keeps dropping to 20-40mb.  If I reboot my modem it goes back to 200-400mb, for a very short while, then drops back down to 20-40mb.  I don’t know what to do.  I have an Arris SB8200 and a TP-Link Deco AXE-5300.  Both are less than six months old.  HELP PLEASE???