Hey all -
I just wanted to inform you that:

New pedals seem to have solved the problem! ...
Upon further investigation, I discovered that if I took the Right pedal in my hand (right knee was worst) I noticed that it actually wobbled from the pedal spindle! Like, REALLY wobbled.
Well.

I got brand new Shimano 540's in August sometime after my first post regarding this.
And guess what? I'm riding more than I've been able to in .. uh, forever!

I attribute this to this unknown problem - no matter what I was having checked out (by docs, by LBS, by whoever specialists in all things bicycles) and no one had ever thought to check the play in the pedal/pedal spindle!

I am so thankful and wanna share this info, just in case anyone ever experiences this - it's one more thing to check. I believe this was always the root of the problem, as the pedals had been bought second-hand after the previous owner had just overhauled them....hmmm?

Anyway, thanks for your thoughts and Happy Riding to All.
I'm Back!