Floating Valves?

"Floating Valves" refers to a condition in which the valvesprings fail to close the valves, allowing them to "float" during the stroke cycle. Doubt that's your problem - floating valves generally result in valve-piston contact with consequent catastrophic mechanical damage. Your problem certainly sounds like a fuel distribution problem to me - but I can't say that for sure. The gradual power loss and the "no smell of gas" on stalling, accompanied by the ability to restart and proceed normally following the stall really point that way to my mind, though. Odd situation though - I think the engine on that truck is fuel injected, isn't it? Dunno if its direct port injection or throttle-body injection - if its throttle-body injection, it could be a fault in the throttle-body's float tank - if it has one (some do, some don't), but if its rail-fed direct port injection, I'm puzzled; that shouldn't be able to happen if the fuel feed pressure is to spec.

A second thought - prolly not pertinent - but if the fuel/ignition systems are tuned for sea-level operation, elevations greater than around 4 or 5000 feet or so can bring about reduced performance. Not at all sure that could explain your symptoms, just thinkin' along for possiblities.

I kind of doubt you would get valve float on a long, steep climb. Usually, valve train inertia will overcome even weak springs only at very high rpms. We can get this on our workover rigs by too much of a downshift on steep hills, which cause the engine to overspeed.

Uh, fuel pressure may be great at the pump, but how is it on the outlet of the fuel filter?

I believe it's rail fed direct port injection and this is happening at a little over 1000 meters. I believe the vehical was assembled in Colombia but with components from Japan. It is extremely puzzling. It occurs only at a fairly steep angle where I'm obliged to use 2nd gear. I have taken it to two different Mazda dealerships here and they couldn't find the problem insisting it didn't happen when they tested it. So it wasn't consistent. Now it is so I guess I'll drag one of the mechanics along again and see what he thinks. The reccomendation from the last visit was to use hi-test gas and change the oxygen sensor (I haven't yet). Could that really have anything to do with it?

Indeed the oxygen sensor could have a part in a fault like that ... and so could altitude difference. Roger's mention of post-fuel filter pressure is somethin' to look at, too - the pressure might check out fine at the pump, as mentioned, but be considerably less at the rail inlet - that'd be the place to check the pressure ... just for grins, might be wise to check the pressure there while the vehicle is on a fairly steep incline. Fuel octane wouldn't have a damned thing to do with your probl4em as reported, particularly as you don't mention knocking or pinging. Your idea of bringin' a tech along for a test drive is good, provided he brings a code reader so he can capture the computer's fault assessment "live", so to speak.