2005 Ford Taurus

On my 2004 Taurus (same everything) the power steering cap has to be pushed down and turned just so to have it lock. I can't imagine it just flying off unless there is something internal going on. Maybe it's overfilled and certain moves causes a pressure surge.

It's really hard to get all the air out of this system, fill then run and watch for tiny air bubbles keep doing this until the fluid is clear. The bubbles gather in the return hose until it has to belch out blowing the cap off make sure the cap if completely seated