Update on this so I don't leave it hanging. A big thank you to everyone - the furnace is running for now!
The pressure switch I had put in a few days earlier, turned out was not working. First furnace guy, on his 3d visit, tested it and found it was not opening at >2" vacuum pressure. Supposed to be rated at 1.13". So I fought with the vendor and got a replacement. He put the old one back in and the furnace worked for him.
About 12/23 the other half called another repair co. without saying she was going to. I was at work, so this guy shows up and guess what, the furnace would not malfunction for him either. I talked to him on the phone and he agreed I was down to the controller board at this point.
I had already ordered a board, which came in 2 hours after the guy was there. I woulda had him come later and install it, but did not know he was coming. :-\ But we were leaving town for xmas, so I quick put the board in after work before leaving, and the furnace still didn't work. We left heaters on and the weather was warm. Got back on the 29th and it was 62 in the house, not bad.
I had also ordered a relay off that board that I thought was bad, but the board got there first.
After talking with my FIL (life-long HVAC guy) I had a few things to try when I got back. I unhooked the air intake pipe at the furnace and cut into it about 4 ft. from the end so as to pull out a section with a bunch of elbows that were inaccessible from outside the house. Blew it out with the vacuum cleaner, no bird nests. Lo and behold, the furnace worked consistently with it unhooked. It draws mostly room air (I kinda have it partially off the fitting so as to draw at least some outside air) so it wastes a bit of heat but it runs.
Either there is some other clog in that 25 ft. of pipe, 20 ft. of which I vacuumed out in the fall, OR...the new inducer fan I put on is actually a bit too weak and does not draw enough air to run the furnace properly. It was supposed to be a correct replacement and looked/fit correct. This could explain why the pressure switch was not opening, if the fan doesn't draw enough vacuum. I think. What I'm not sure about is why it works sometimes. Might be wind against the house helping the intake sometimes, or there is a bird nest I can't see that flips up and down sometimes and blocks the air flow.
Soon as the weather clears a bit I'm going to get my leaf blower out of storage and blow out that 20 ft of inaccessible pipe good and hard. If that doesn't find anything I was thinking of putting a T and valve on the intake line near the furnace so I can bleed in enough room air to make it run.
I don't think the furnace has too many seasons left in it, the heat exch will probably rust out soon. I've spent about $500 in parts and labor so I'm not chucking it now!