Reversed the inlet/return lines and car ran great. Went for a highway drive, 70+ mph was no problem. Held idle great too. Put in a bottle of injector cleaner in the tank (not lubra moly, but what Advance Auto had). Drove the 20 miles back home at highway speeds and was tickled to death! Left car idling for about an hour to run injector cleaner through injectors. Turned car off for about an hour.
Still on reversed fuel line set-up, I went for a grocery store run...engine slowed, nearly shut off. Tranny in nuetral, foot to the floor and no throttle response other than idle. Throttle response finally resumed after about two minutes, was able to get home.Same exact symptoms I had before I started this fiasco 3 weeks ago.
No crap in primary filter that I can see and primary filter is full of fuel. Still plenty of fuel in tank to run the reversed fuel line set-up, so I'm not out of fuel.
Now I know it's not a plugged up tank screen, not a bad lift pump, not a plugged up ALDA, not a plugged filter.
New suggestions? I'm guessing it's down to injectors or injector pump?
I just can't figure out why this happens sporadically.

Try hooking a bottle of diesel in the engine bay. It eliminates the rest of questions of the fuel supply. Sorry to say but it may be the IP. The injectors would not fail all at once.

I'm guessing IP as well.
Is it odd that the car ran extremely strong at highway speeds then reverted back to a no-run condition? I would think if the IP failed, then it just failed, no intermittent problems.

I would install a fuel pressure gauge on the line between the main filter and injection pump to make certain fuel pressure is always present. Anything over ten pounds would let the engine run easily. You may be periodically loosing this pressure. Gauge 0-30 pounds liquid filled is ten dollars at disount places.

Dump some Startron into the tank before pulling the strainer. West Marine sells it.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

I could not get my head wrapped around the concept that the injector pump would work normally for a lengthy drive and then not function at all.
As with all WVO conversions, there are several connections and hoses for the solenoid selectors. Just prior to the IP I had a "T" in the fuel hose to allow for a temperature sensor to monitor fuel temps when on WVO. I decided to unscrew the sensor, clean the T and plug the former temp sensor threaded opening. Viola! Apparently my temp sensor was leaking just enough air around the fitting that at higher rpm's the engine could handle the small amount of air, but at low rpm's the ratio of air to fuel was just too lean and the car would eventually stall. I think there may have been some effect of the heating/cooling of the fitting that also allowed temporary bursts of normal operation, but the idle was never quite right since the beginning of this problem.
I did find the dreaded "chicken skin" in my WVO lines, but I will tackle that clean-up next weekend. I will need to post some info on that to the veggie guys on another forum.
For now the car runs perfect, no issues.
While chasing these automotive gremlins, I often become frustrated. However, because of the elusive gremlins that cause us to search and probe and tear apart, we find so many other things that are in need of repair along the way. That's a good thing in the long run.
It didn't cost me a lot of money, just a lot of reading and wrenching. Like my wife says..."If old cars were easy to fix and cheap to own, everybody would drive one."
Thanks to all for your input.
I will post back and bump this thread should there be any further happenings.