'85 E-150, Sputters Under Load, Throttle Up

[Worktruck]

Greetings All,
I have an '85 E-150 with the 300ci straight six. There are approximately 135k miles on the drive train. The thing has always run like a champ and used to be my number 1 vehicle, but due to a long drive to my job I needed to find something with a little better gas mileage. The van is now a work truck. I was running it for maybe a half hour or so every two weeks to keep the battery charged and driving it for maybe 15 miles or so once a month.

Now to the problem. I used the van to help my sister-in-law move into her new place. The van ran GREAT, loaded down with a full load of furniture, no problems at all. On the way home I was going up a slight, long grade when the van started sputtering almost as though it was going to quit on me. The more gas I gave it the more it bogged down. I pulled off to the side of the road and just let the thing idle for a few minutes (I had the ECM quit on me a few times, I thought maybe this is what was happening, but come to think of it when they went before they just DIED dead. No sputtering, engine just shut off). The engine never cut out and I was able to drive home without any sputtering. This included another hill longer than when I noticed the problem.

When I had a chance to work on it I took it out for a ride and sure enough the sputtering was back. I replaced the distributor and rotor caps to no effect. With the tranny in park, if I get on the throttle a little bit the thing sputters as well.

Anyone go through this themselves and find a solution?
Thanks in Advance!!