Doesn't really stall but engine goes to idle while pulling hill
 

This is really weird!!! A recent fishing trip to Montana pulling my travel trailer up grades cruising along at 50 mph in 3rd and the engine just goes to idle like I let my foot off the gas. Did it several times. A couple quick jabs to the throttle and we're off and running again.

Never did it all the time nor on the level. Ford could find nothing wrong after keeping it 2 days.

What's the build date on your truck? There's a TSB out there for the transmission that may alleviate what you're experiencing.