Reader DC Tools to Select or Highlight text not working (or only sporadically)

I am using Adobe Acrobat Reader DC v "2015 Release | Version 2015.020.20042" on a Windows 7 PC.

It has worked great forever but has developed, in the last couple months, an annoying habit where certain tools seem to 'check out' and simply do not work. They may start working again much later (after a reboot, or just passage of time, or due to some other software change?). They cannot be forced to work again by just exiting the program and restarting it.

I can find any pattern to why it starts nor why it stops.

It affects many documents (and all documents the same, once the issue begins). It is NOT a matter of me trying to highlight an image (rather than text) - I can be successfully highlighting text in a long document and then suddenly I cannot (except one word)... same paragraph.

It is NOT that I think I'm using Acrobat/Pro - I know what Reader is and its limitations. But this feature works sometimes and not others.

Affected tools I've seen just not do anything anymore: Hand (pan), Select Text (can only select one word -by double-clicking), Highlight.

I have tried to "update" the software and it reported being up to date.

I have completely uninstalled the app and re-installed it from Adobe's website - and the problem still re-appears.

How can I address this and make Reader predictably functional and useful for annotating again???