Started my truck up to go to work this morning and about two or three minutes into my commute I lost nearly all power. Was less noticeable on level road but when I started climbing a significant hill about 5 mph was the maximum speed I could get, pedal to the floor. 30 mph was about the max I could get on even a slight incline. Check engine lights was on when I looked at the dash.

Pulled over and turned off the truck for about 30 seconds. Truck started fine again, idled fine. Full power after restart for only about 20 seconds then lost it again. I returned home and parked it for about 4 hours then drove it to my mechanic. Still no power. Mechanic not home so I went back home. On the trip home the problem seems to have went away. Parked it for another 2 hours and drove again, no power problem. Issue seems to have gone away. Check engine light still on. Don't have a reader.

Any ideas what might cause this sort of thing?

Sounds like it's in limp mode. I bet your check engine light is on for the air injection system. Search the forums for it...there are no less than 1 million threads about it. Sounds like a bypass kit is in your future.

Second what Skott said. Wanna say its the pre 06 models that had problems with this.

That seems like it. Picked up a code reader from walmart and it's P2440, Secondary Air Injection System Switching Valve Stuck Open Bank 1.

Limp mode seems to have cured itself so I'm not sure if this is going to be a problem that needs addressing.

The valve/s most likely became unstuck, the valve/s get stuck because a bit/s of the plastic impeller in the air pump has broken off and gets stuck in the valve/s, if it became unstuck, you're lucky because that usually doesn't happen often, BUT err on the side of caution and GET the bypass and install it and you'll never have to worry about this happening again!!!