I'm having a tough time figuring out what's causing my Vanagon troubles. This is a 1987,  manual trans, engine is believed to be original 2.1 with just under 130,000 miles. Here's what's happened so far. Started with one problem and it morphed into another. Doug in CT helped me all of Saturday to swap out parts, but there's still trouble.

I can go down the road and get up to highway speed. Then it loses power, in any gear and can barely get out of its own way. BUT if I shut it off and start it again, I have power again for another cycle. 

The problem started out a little differently. At first, it wouldn't idle right until it had been idling about 15 minutes, then it was fine, without the loss of power problem. Actually, it didn't have power until it had idled for that long. 

We changed the AFM, and it idled fine, and I could take off immediately at highway speed, once, then it would lose power. Also changed the throttle position switch, which was bad, too.

We tried replacing, one at a time, the ECU, fuel pressure regulator, ignition coil. I tested the output of the fuel pump per Bentley, and the output is double the minimum required. Also tried disconnecting the oxygen sensor, no difference. 

The only thing I can think of and wonder about is the oxygen sensor. It's the two wire one, and there's another wire in the sheath that looks like it's connected to ground. This is definitely not original.

The engine was out when a mechanic replaced front and rear main seals and the clutch.

Any ideas? 

Thanks, 
Ed in CT