Hi there

From your description, it certainly appears to be a Wifi issue - You could confirm this by plugging in a LAN (ethernet) cable to the TV from the router - but as the TV is downstairs, it would mean lumping it upstairs to do so.

However, the solution will be probably be in the form of  "Homeplugs".  They are available on Amazon : HERE - this will provide a "wired" solution.

Cheers