What happens if you update to the newest version from http://getthunderbird.com/ ?

Update does not fix. Deleting Thunderbird and reinstalling does not fix.

This is on a Surface Pro. Took unit to Microsoft, they did a "deep clean" removal of TBird program and reinstall. Did not fix.

Do you have any security software that might be accessing the same folder as your mail is located? For the index to go bad repeatedly I would be looking for an add-on that did not make the transition to the latest version without issues or an external product that is interfering with the folder while it is in use.

The most common of these external programs is anti virus products. But having the profile in a folder that is synced, either through backup or to a cloud service can also cause contention issues that result in index issues. I am seeing more and more people with issues caused by trying to synchronize profiles across devices using cloud drives

Thanks, but I do not sync this with other devices, software or cloud. I do download email to my phone which not an issue.

I bought a new computer; installed TBird; transfered all the files in my profile folder from the 1st computer and copied into my new computer TBird profile. The problem persists and is identical to the first computer.

If I have a security software accessing it, it would be the result of an up-date that I am not aware of. I am not enough of a geek to know if the anti-virus is a possible disruption. But, my gut tells me it is not.

DRHarts said

I am not enough of a geek to know if the anti-virus is a possible disruption. But, my gut tells me it is not.

What anti virus are you taking about. I have to use generalities because I know nothing of your system. Generalities from your do not provide any useful information.

We need more than Gibbs gut, we need information to do more that guess.