I have this Mac:
And it's attached to an Acer P191w. It will show an odd grid of dots on the external monitor like so:
When I move the application the dots will get 'refreshed' and go away (they're not stuck pixels) but will quickly 'refill' themselves in 5-10 seconds. The app they're on there is Sublime Text 2, but it occurs on any app. I just put it up for contrast. I don't appear to get the dots on my laptop display, just the external.
I have checked the wire connecting the two, it appears tight and secure.
Occasionally this will prelude a full lockup of the machine forcing a hard reboot.
I'd appreciate any insight into this matter.