Wifi connection drop off

Why does my Firestick 4K max after doing a restart and then a speed test right away run 315mbs speed test BUT after a minute tests 90....seems it connects real strong always and then goes to weaker connection. I am connected to a 5GHZ network, closed all background apps, cleared the cach etc etc....I have a Gig internet speed and my router is right beside my tv.

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