UP2716D problem -- current input timing not supported. Help please!

Monitor occasionally goes to black and then back (after a couple seconds). Then every once in a while a notice comes up saying:

The current input timing is not supported by the monitor display. Please change your input timing to 2560x1440, 60Hz or any other monitor listed timing as per monitor specifications.

This monitor has worked perfectly for months with this same desktop with a previous owner. I don't think any settings were changed.

Connected via HDMI.

Thanks.