My slant six truck has had a number of issues over time, but now I'm facing something that is so confusing I'm baffled. I've had issues with pinging that I think I have a handle on, several interesting electrical glitches, etc, etc.

Right now, I have a problem that just does not make sense. It's something that occurs solely at WOT, and a specific rpm, that's about 3000 - 3200 (don't have a tach) and what happens is slow rythmic surge. It's not an ignition interruption, and it goes away if I let up the throttle up part way. The surge will just fade away and it'll continue climbing rpm. If I get the rpm past that point, it pulls again, with no surge and no missing. It won't do it in a low gear where the engine changes rpm fast. It does it only when pulling hard in 3rd gear where the rpm changes slowly - when it hits that specific rpm, it begins to surge, which is slow rolling feel, it seems to gain and lose power, but not violently, and it doesn't really seem to missfire.

It's almost like the carb is alternating rich/lean in a slow rythm. When I changed air filters and cold air intakes, how it behaved changed completely, changing rpm and also going from a fast jerky surge to a slow rolling one.

Any ideas? I'm now baffled.

Reminder now, it's a propane engine, 10.5:1 CR, super six intake, CA 200 mixer, Ford EDIS crank trigger ignition, running a Megajolt controller.

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'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.