The current input timing is not supported

I have a Dell XPS8500 with a Dell ST2420L monitor. NVidia GeForce GT640 card.

Intermittently I get a message on the monitor -

"The current input timing is not supported by the monitor display. Please change your input to 1900x1080@60Hz or any other monitor listed timing as per monitor specifications"

Computer may work fine for a few days. Next day the problem may occur within 5 minutes. Other times computer may run for a couple of hours before the problem occurs.

I have checked and replaced the cable - problem still occurs.

I have updated the GeForce driver - problem still occurs.

I have reset the monitor settings - problem still occurs.

I have updated to Windows 10 - problem still occurs.

Any suggestions welcome.

Thanks.