I have a 1985 dodge conversion van with a 318 with a severe power loss. It has mostly sat for the last 2 years other than occasionally starting once every other month. I recently took it out for a camping trip, and it ran fine. On the way home it ran a little rough, then 30 minutes later I felt a power loss. It got worse as I drove it. Eventually it was so bad that I had to limp home with the flashers on going about 5 mph.

Symptoms:
Starts right up and idles fine. It dies when I put it in gear. I've had the transmission serviced a couple years ago, so I don't think its that.

It also has a habit of stalling out after driving a long distance on the highway. Is it not downshifting?

I plan to start checking the obvious stuff, but would like on input on what to check first. I've already done the plugs and wires.

Fuel pump? It starts right up.
Catalytic converter? How do I check this?
Carburetor problem? what should I look for?

Thanks
Justin