Can only copy and paste sporadically

If you like to use ctrl-C and ctrl-V this could be a problem with your keyboard. It is always good to upend the keyboard from time to time and without the actual keys in contact with anything, a few sharp bangs on the underside often produces a small pile of crud.
 
This is a shot in the dark but easy to try, try clearing the clipboard. Google clear clipboard for your version of windows to learn how (or empty clipboard) Once it's cleared try copy/paste again
 
Not pretty, however...

... gbhall gave you very sound advice.

As your problem is intermittent, IMO, it's unlikely that software is causing the problem. So, Yes: Bang, smack, whack the keyboard a couple/few times while holding it upside down and at varying angles.

You did not specify which you're using but, of course, if its a desktop's keyboard, you can use more force safely than with a laptop's. If it's a laptop, in order to minimize the "trauma" to the whole machine, you may wish to go to the trouble of removing the keyboard entirely then, (gently) bang, smack, whack it on the plastic-film-covered rear.

When my laptop or desktop keyboard acts as does yours, I follow up the above with a very liberal spray of alcohol to the face of the keyboard and then immediately lay it, upside down, on a paper towel for a minute-or-so. This "unglues" that long ago drop of sugary coffee I splashed!
 
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