Solved! It was the fuel pressure damper, located under the upper intake manifold at the fuel rail where the fuel line enters. Must have been plugged. Replaced that and now it runs like new. It's a rare problem and was the LAST thing I suspected. There had been intermittent fuel pressure but it seemed OK after the fuel pump and pressure regulator were replaced. Checked pressure again after lots of fiddling around, and found it to be low again! Then I knew that there was still a blockage somewhere. It was expensive, but I'm glad to be driving again and not riding my motorcycle in the cold.