Do you see the 'not responding' message in the title bar?

What is your anti-virus software?

First it will lock and then I will generally see the 'Thunderbird not responding' msg. I use Trend Micro Maximum Security for anti-virus.

 Thx!

Try to start Windows in safe mode with networking enabled. - Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

Still in Windows safe mode, start Thunderbird in safe mode. - https://support.mozilla.org/kb/safe-mode

Does the problem go away?

ok. I might try running Thundebird by itself in safe mode first. If that doesn't work I'll go windows in safe mode and also thunderbird.

Also, for what its worth, associated with this happening I see many misc folders called 'popstate-N.dat' that appear under the trash. They are empty, and if I delete them they come back the next time the issue occurs.

well Thunderbird safe mode seems to have helped but not completely fixed it.

It went for awhile without it happening - thought it might have corrected the issue.   But finally an email came in and I clicked on it to read it - locked up...came back a bit quicker than usual (after about 20sec) but still locked up. 
I guess I'll try the Windows safe mode/w networking  as well.

I tied safe mode for both. Didn't see any lockups but I only ran that way for about 4hours. Normally I'd see it in that time frame - I can't say for sure if that means anything because its the weekend and I wasn't receiving much email (nor sending it). I suppose I can try it during the week - though I can't do it for long as some of the basic programs I use, like WORD, won't kick up in Safe mode.

Windows safe mode disables anti-virus software, which is most likely the culprit.

Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

Keep it working. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

And last but not least, backup your Thunderbird profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

I was able to put the exception for the profile into the anti-virus sw. I also found the button to turn on/off scanning attachements (was already off). I can't seem to find anything on scaning incoming/outgoing message though or on intercepting secure connections to the server.

It happened again though. Seems to be related to activity of receving a msg and/or while sending one.

I'm going to try running in Thunderbird safe mode again now that the profile scanning is in the exception list adn see if that helps.

Any other ideas appreciated. thx for you help so far!

If there are no problems in Windows safe mode, and running Thunderbird in normal mode anti-virus software is most likely the culprit. If you're unsure how to implement the suggestions above ask in a forum for your anti-virus software.

If there are no problems running Windows in normal mode and Thunderbird in safe mode, the problem is most likely with one of your add-ons. You can try to disable them one-by-one until the culprit is found.