After replacing everything inside the toilet tank, I turned the inlet water supply on and nothing happened. I disconnected that line from the toilet to run water into a large pan. All I got was a lot of noisy chattering (air lock?) but no water for a few seconds and then the water started running along with the chattering. When I turned the valve further open, the chattering ceased and the water ran good - BUT - when I closed it down, the chattering returned at the last moment. This scenario repeated itself several times before my patience ran out.

Replace the stop valve, what you got was not air but a vibrating washer in the stop valve.
Did you inadvertnetly post on Billythe3rd's post? If so please delete.

Yep I was going to say I have never seen an air lock in all my years but sure read a lot of questions about them. That is an old style stop cock with a washer. If you can't tighten the screw on the washer just replace it with a nice new 1/4 turn angle stop cock for about $5.75 plus teflon tape.

Those quarter-turn valves are worth every penny of $5.75. They are so much easier to use that I would replace it just for the advantages of the new valve.