This is one of (many reasons) why you shouldn't try and achieve your goal by way of UI scripting. I'd suggest, where possible, going through each of your scripts/macros and rewriting them using a solution that doesn't involve System Events in this manner.
I experienced the intermittent behaviour you describe using System Events when I switched to Mojave. The solution I found was to quit System Events right before using it for some purpose. When it's quit, it immediately restarts, but seems to work reliably for the remainder of that script. So in your case, an example would be:
tell application "System Events" to quit
tell application "System Events" to keystroke "c" using {command down, option down}
I assumed this is a Mojave bug, but it may help even if you're not using Mojave. I also agree with CJK.
An option would be to simulate the keys with Keyboard Maestro. You can use the do script AppleScript command to execute the XML of an action, and you can use the Edit ➤ Copy as XML to copy the XML of the Type a Keystroke actions.
thanks Peter, this seems to be a very interesting solution, I see how I could do it with a KM macro, that has the "type keystroke" commands and is called from the Applescript with the "do script" routine, but I'm not quite sure how to implement it with the XML. Can you give me more details?
The XML code goes on the applescript itself? Please forgive me for the newbie kind of questions and thanks again.