Corrupted text when composing Apple Mail messages - Virus?

In the last two weeks, I have started having bizarre corrupted text issues while composing emails in Apple Mail on my MacBook Airs (I have two - one at home and one at the office, and both have the same issue). Here are the details:

When I am composing a message, part or all of the previous sentence seems to disappear and be written over by my new text. If I wait long enough - or later move the cursor to a further point in the message - the original text usually reappears, but the timing and circumstances are not consistent. This means that I am never sure whether I need to re-type the missing text or not.

This happens sporadically but often - maybe 33% of the time. It happens on various accounts (dot.Mac, gmail, outlook). Both Macs are running 14.3.

It is driving me nuts, becoming a drag on productivity. Is this a virus? A keylogger someone installed? What else could it be? Thanks.