Losing Power, Stalling and Popping On Straightaways and Climbing Mountains

If the truck has sat it could have bad gas in it and it turns into a varnish like substance which will clog fuel filters and injectors. The other posibilities are intake and exhaust tract restrictions . When going on short trips the exhaust will not always get all the water that condenses in it out and it will rust. Check the intake tract and then check the exhaust. Mostly look at the catalytic converter. They can rust inside and fall in on themselves and look okay outside. Bang on it with hand or rubber mallet and listen for loose material.
I am going to give you a diagnostic walk through as well. It goes into a lot more detail than I can here. Just follow the links below;

http://www.2carpros.com/articles/engine-misfires-or-runs-rough

http://www.2carpros.com/articles/engine-has-low-power-output

Thanks! Come to think of it, I bought gas in middle of trip. I didn't have problems before that. It's very possible they sold me bad gas, which is not uncommon here, unfortunately. Neither food nor gas is tossed here when it becomes bad. It sits on the shelf or in the tanks until it is sold.

Would that make sense? Buying bad gas? Again, I only had problems when climbing mountains and speeding down straightaways. When I tried to rev the engine, nothing happened except I heard the popping the sound.

Actually, my wife used it today for our normal short trip into town. She said it lost power at a traffic light, so she restarted it and was able to go.

So still wondering if/hoping it is bad fuel from last fill-up. If that is the case, should we hastily drain the fuel? Or will it be OK once this fuel is gone? Should we add some kind of octane booster or fuel cleaner to current tank?

No i would not drain gas. You might want to change the fuel filter if the truck has an inline filter. If the truck sat the gas turns to varnsih quicly and once you run the truck it can then clog up thing like the fuel filter.
Otherwise the diagnostic walk throughs will cover all the points to help you find the problem.

We have a 1999 4.6L F150 I bought over 10 years ago up in Ohio. We live in Maryland and it was the only reasonably priced 4x4 extended cab I could find at the time. Other than being rusted out from Ohio's bad winters and heavily salty roads, it ran decent for the first year we owned it.
After the first year, the transmission never had issues but the engine started running poorly and had excessive "popping" noise under the hood with extreme loss of power under heavy loads- hauling anything from our camper trailer to lawnmowers and lumber or firewood with our landscaping single axle trailer.
I replaced as much as I could on my own to give it a good tune-up with the best quality parts I could find: K&N Cold Air intake, NGK spark plugs and MSD coil packs as well as a new OEM ford fuel filter and ran Gumout FI cleaner in the tank as instructed. It still continued popping under heavy loads so I took it in to a reputable, honest auto shop I often use called Martin's Garage in Maugansville, MD which is run by Mennonites. They diagnosed it as a cracked exhaust manifold which is common for these F150's.
Took the truck back home and ordered a BBK exhaust header set from autoanything dot com. They arrived and I broke two of the existing header bolts trying to remove the stock header which was cracked, so I took it back to Martin's garage and they welded a couple nuts onto the broken studs to have them removed. With the new BBK headers, the truck ran better than any standard brand new F150 right off the lot. So we then proceeded to order a stainless Flowmaster full dual exhaust kit and the truck really opened up.
I've replaced the spark plugs at least four times now since then with an additional 100k miles added, but have noticed the one plug directly under the power steering reservoir gets fouled first before any others, and when that happens the engine runs poorly again. I should get 20-30k miles out of each set of plugs, but perhaps the way we haul heavy loads quite often- I'm having to change the plugs around every 10k miles. The best ones I've found are not the NGK Iridiums I started out with, but the cheaper Autolite $2 a plug Part No. 764 (check the user reviews on advanceauto dot com) for our truck. Hope this helps someone as you many not think it's the spark plugs since you've recently changed them in the last 10k miles, but if you take them out and see one fouled out..it may be time to change them and see if your power's restored. Now we keep at least one set of 8 Autolite 764 plugs in our truck glove box ready for anytime it starts acting up. The truck runs strong with these upgrades and it sure beats any truck payment. Good luck!