I removed the filter temporarily to eliminate the possibility that it is blocking air flow. This made no difference.

Yesterday I replaced the (combination) fan and limit control.
No change.

Today I tore apart some flue sections to see if there were any blockages, dead squirrels, etc. Not much soot for 35 years, the chimney is also clear.
Still having the same problem.

BTW, are furnace fans usually 1 or 2 speeds ?
Does anyone from appliancepartspro ever read and respond to these questions ?
I was told by the (somewhat rude) live chat person to post here.
The person that answered the phone said basically the same thing, that I would receive a technical answer within 24 hours.

Thanks for your reply.
It was indeed the fan motor, which I diagnosed without any help from APP.
BTW, are you an appliancepartspro employee ?

For anyone else who gets down this path;
At some point it occurred to me that, but for the new switch, the motor was hooked pretty much directly across 110 VAC. Since I had already replaced that switch ....hmmm, kinda/gotta be the motor, so I did in fact wire it directly across 110 VAC (with the shutdown switch set to OFF) and sure enough I got the same symptoms. The whole lower blower unit is mounted on rails that slide out after removing a couple of small retaining screws. The squirrel cage is mounted to the (1/2 inch diameter) shaft with a set screw to the milled flat, the motor is mounted to the opposite side of the housing with 3 bolts to arms that are held to the motor with a band. All of this is quite intuitive once you have it all out. The 3 arms and retaining band had to be transfered to the new motor, of course I took care to be sure that I got it on STRAIGHT, tight and as close as I could to the same distance from the motor face as it was on the old one. I took the old motor with me and picked what little brain I could find in the local sales counter staff. Well, it came with a schematic anyway. The new motor came with a longer shaft, which didn't matter because it just sticks farther into the fan cavity. I also turned around the 4 thin screws that run through the motor and hold the end plates on, to get their excess length away from the face of the motor where it MIGHT have mattered.

I now (for 2 days) have a very quiet and seemingly efficient heating system. It cycles more evenly, I suspect partly the new motor and partly the cleaning that I did are responsible for this.
About $200 total for switch and motor, the neighbor paid some $5500 to replace his about 5 years ago (same symptoms), but I only found that out yesterday (-:
What to do with the $5300 I saved ?
I'll think of SOMETHING I'm sure, some of it should probably go to converting it to pilotless ignition (hot surface), something I've thought about every few years as the co$t of natural gas has continued to rise.

If I could swap out the burner & ignition system for a higher efficiency unit that would be fine. If I had to replace the whole standing unit that would be impractical. Just getting a new unit in and the old unit out would require the moving of too much "stuff", walls, etc.

I could get 50 or 60 pound sub-assemblies about the size of the blower unit in there without too much trouble, but not a whole cabinet.

If there isn't a "guts replacement/upgrade kit", maybe there should be.

REALLY ?
Six YEARS later and all you can contribute is the standard noise from the generic owners manual ?

It worked FINE for 35 years, WITHOUT any so called "service"
a PART wore out, it was replaced,
it has worked FINE for 6 more years, WITHOUT further so called "service".

Evidently I do NOT need a "Professional Furnace Repair Service" - and I seriously doubt that many other people do either.

A SIMPLE, a VERY simple DIY repair of a household item.
"Professional furnace repair service provider" indeed.