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I've got an 89 , v6, auto...
This is what happens: When I tow my trailer with my two snowmobiles on it, it will lose all power, and you have to feather the gas to keep it running. It's fine for the first few miles and then it starts doing this odd little thing.. It will act like it hasn't any power for about 15 to 20 seconds and then it comes back to life, then a few more miles down the road the problem happens again, and repeats. I thought it could be the fuel filter or pump (new one two weeks ago), something electonic, or something with the transmission and/or the computer.
It runs fine when the trailer is off, and I can get it up to 70mph without a problem and everything is fine, so I'm thinking it's not a fuel issue... since going 70 w/o the load would use as much, if not more gas as doing 35mph with the load...

Any ideas? Suggestions?

That's a good one. I had a similar issue with my '88. Changed out everything it seems, but it ended up being the 02 sensor.. didn't even show up on the computer.. BUT yours runs without the trailer.. very strange.

My first though was do you have a short in your trailer harness? Then I seen you have an auto tranny, towing is hard on any vehicle so I would suspect your transmission. Do a fluid and filter change if you didn't already, add some lucas transmission stuff. The auto's in these rigs are not known for being the strongest so I'd look at that first.

when you replaced the fuel pump did you replaced just the electric pump or the whole plastic assembly. my 90 5-speed would do the same thing, i took the plastic piece in the tank apart and put a new pump in the old assembly, after 2 months i replaced the whole assembly and fixed the truck.