Towing Problem

[fasted64]

I am having a problem towing my boat when going up hills. I have had my 2000 F150 4x4 in the shop several times and they can't find a problem. The problem is that under heavy load, my truck just will not move. The engine loads down, but the tires will not spin. It happens in all gears, but it seems to be a different gear each time it happens. It has happened 4 or 5 times in the past 6 months. It is almost like applying the brakes and then hitting the accelerator. The engine loads, but the truck doesn't move. It's not the brakes though as the truck rolls just fine. The boat trailer seems fine too as I can easily roll back down the hill when the problem occurs. Generally, switching to a different gear will fix the problem and I can get the truck moving again. The most recent incident was in reverse. I had no choice here on switching to another gear as there is only 1 reverse. It took me 5 minutes of switching in and out of gears and in and out of 4 wheele drive and in and out of neutral and park, etc. before something finally happened where the truck would move again. When it did start to move again, it was very sudden. All of a sudden, the tires just started spinning normally and I was moving again. Note: This only happens when towing and only under a heavy load like going up a hill. Temprerature also does not seem to be a factor as the truck was cold on this last incident. My question is does anyone have any other idea on what could be causing this? My guess is it's either the transmission, transfer case, torque converter or some other electronic thing that controls the whole drive train. If anyone has any ideas, I'd appreciate it.

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[Bluegrass]

You got a new one on me.
The only thing I can think of is possibly the timing plug or line is intermittant. Pull it out and back in several times.
If this line goes open, the engine loses all advance hence no power under load.
Just a long shot guess from the way you say it acts.
There is a small chance it is the converter one way clutch but you should get a CEL lite from that, at some point.