red green dots flashing all over the display, Dell U3219Q

Hi @james2212

To me it looks like some sort of signal interference is occurring, noise perhaps.

Initially try the built in diagnostic tests as seen on p.15 of the service manual and check if the problem occurs on one or all of the tests.

If it occurs on all tests, you may have to open the monitor and visually inspect, initially the main board and power board, for any obvious signs of component damage e.g. burnt out or heat stressed components. The manual describes the disassembly procedure starting on p.8. Here's a disassembly guide that has more detail. Presumably given that it shows removing parts from the mainboard, it is more of a monitor disposal guide I think. You could also check that all the internal cable connections are secure as well

If it all appears to be OK, unfortunately I cannot find any schematics for the boards so depending on how much you want to experiment (and pay) you could start swapping boards i.e. a replacement mainboard is ~$US40-50 and a power board is ~$US25-30