07' D12 Fuller Eaton automatic slips shifting

When driving up to 6,7 and 8 gear; transmission starting to have hard time to shift or gets stuck in shifting and my gas pedal becomes unresponsive.

Press gas, and nothing. No engine revving etc. Truck just rolls in whatever 7 or 8 gear it's stuck.

Sometimes after manually shifting the "up" and "down" gear selector buttons, it starts working again.

Most cases I have to shut off the engine and start it up again and everything resets.

This is starting to happen more and more often. Sometimes driving on hwy 55-65mph truck will suddenly lose the gas pedal and eventually engine will just cut out going 50mph+ leaving me coasting at high speeds with no power steering.

Have to pull over - coast to a stop. Wait a few minutes. Start it up again and get going.

This is becoming more and more frequent.

What could be the culprit?
Where do I start?