Well I hope it is the final episode...
Finally made some progress. Come to find out it was a clogged line to the city sewer. Or rather a partially clogged line.
After some careful observations, and awakening some long forgotten memories, I discovered if there was any water visible looking down the clean out, the toilet would not flush. Once the water drained down out of sight, the toilet would flush.
Evidently there needs to be free air flow down stream, or the toilet trap will not siphon!
Going back about 10 years ago, I was having slab settling problems. Hired a local company to dig down and pour concrete jack pads around the perimeter of the slab, raise the slab back up to where it originally was. One of the requirements was that I hire a licensed plumber, $300 (and they just so happened to know one) to leak check the drainage system under the house. There was only a single cleanout port, and there needed to be a back to back cleanout for them to do the test. They could install that for a nominal (astronomical) fee. I declined their insulting offer and put the cleanout in myself.
That's what jogged my memory, that there was not any water standing in the pipe at that time.
So here's what I finally concluded:
There was a build up somewhere near the end of the pipe, possibly where the line connects to the city sewer.
I took a walk up the block and found a man hole cover marked 'sanitary sewer'. It was in the opposite side of the street from where I assumed the line was. Also the neighbor stopped by while I was outside this evening, he said that he watched the city dig that line up once, and yes it is on the far side of the street and also about 10 feet deep.
That means the snake I rented was not long enough to reach the end of my line!
Yesterday I was telling a coworker about my plumbing perils, as he had had problems too. He offered to loan me his pressure washer attachment, a high pressure line cleaner that blasts clean the inside of the pipe.
So I bring it home this evening, pressure washer won't start, cleaned the main jet, got it going.
Started running it down the line. It self feeds the way the water jets out and back. I very slowly fed it in, pulling it back and forth, letting it slowly progress forward. It did feel like it hit some restrictions, but hard to tell, it may have just been the drag of the hose the further it went in.
The hose was supposed to be 150 ft long, plenty to reach the main line, and I pulled it out across the street just to be sure it was long enough. So I feel sure it reached the end of the line this time.
So far all looks well, time will tell!
Thanks everyone for your input!