many thanks to those who have offered help, and I think you all had the right idea, that there wasn't enough airflow.
when I last closed down, I took the top and sides off the tower and I noticed that the power supply felt uncomfortably hot.
half an hour later, I rebooted the machine with the top and sides off, (that was three hours ago, and there hasn't been a flicker on the screen display since then, despite having been accessing YouTube constantly during that time ).
ergo, there wasn't enough airflow (although why it should suddenly have become a problem beats me)
so my intention is simply to leave the top and sides off, and as the tower sits under a desk, there shouldn't be any problems.
every so often I will take it to my workshop and give it a good blow out with compressed air, to remove any accumulated dog hair etc ...