New 3.4.2 not highlighting

Good morning. I just upgraded 3.4.2 a month ago, and all was going well with editing my recordings…until last week, when the highlighter suddenly would only take large sections at a time. It’s seemingly incapable of highlighting and deleting any small sections as the cursor has a mind of it’s own. Anyone else experiencing this, have I accidently turned something on/off, or…???

Thanks for the tips
TL