Steering Wheel Shaking

I recent bought this 2007 Honda Crv, the steering wheel sometimes shakes when on the highway when going over 50mph. It's happened about 12 times at random this past month and i don't understand what it is. The TPMS light is on and i'm not sure why because they all have enough air. The shaking lasts from 20 seconds- 1 minute. Mechanic isnt sure what it is because he just aligned them.

When pressing on the gas it shakes but taking my foot off the pedal makes it shake faster. Any help would be so so appreciated !