Why is my Wii U having massive fits of controller lag?

I bought a new Wii U a few months ago. I most often play Mario Kart Wii (in Wii mode) and have been noticing an increasing amount of of controller lag.

I know that the Wii isn't known for lagless controllers, but this isn't typical .1 or .2 second lag for all input. We'll be playing fine for twenty minutes, then all of a sudden the Wii will have one of these "fits" where all controller input lags and becomes extremely choppy. Pausing the game and waving the Wiimote slowly in front of the TV reveals that the Wii is picking up inputs only 3 or 4 times a second. (i.e., the hand icon stands still and jumps back and forth in a choppy manner.)

Re-syncing/resetting the Wii U stops the problem, as does waiting it out sometimes, but it makes many games completely unplayable when you suddenly lose control. What can I do to fix this broken Wii U?

You have significant wireless interference.

The Wii Remotes use a Bluetooth wireless connection, which is a 2.4 GHz frequency. The problem is that many other devices use this same frequency including wireless phones and wifi.

The Wii U GamePad uses wifi technology in its own way, and you could test its effect on the controllers by entering Wii Mode as TV Only, leaving the GamePad off.

Other things to try would involve unplugging wireless phones, keeping cellphones away from you, making sure no one is downloading or watching something via wifi.

Short of playing within a faraday cage, you will never be free of competing wireless signals. The Wii U can handle some noise, but it can be difficult to manage if neighbours' signals are out of your control.