Ask Different is a question and answer site for power users of Apple hardware and software. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I have this Mac:

About my Mac

And it's attached to an Acer P191w. It will show an odd grid of dots on the external monitor like so:

Odd dots

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.

share|improve this question
Have you tried a different external display to see if the issue persists? – l'L'l Jan 23 '12 at 0:04
Alas, I only have one external display :-/ – Alex Mcp Jan 23 '12 at 0:24
up vote 2 down vote accepted

A discussion thread on says this can be fixed by unplugging and replugging all of the video connections.

share|improve this answer
Link? (and thanks!) – Alex Mcp Jan 23 '12 at 0:49
I added the link, but it doesn't provide much additional info. – bneely Jan 23 '12 at 0:53

If you can see it in a screenshot, it's likely something with the video signal being sent from the GPU. This could be a problem with the GPU itself or some kind of weird software issue. The best way to tell is to try booting to an external OS like your recovery partition or an OS on an external hard drive to see if it occurs there as well. If it does, it's probably an issue with the GPU. If not, try a reinstall. If you don't have these resources on hand, take a trip to your local Apple Store to have them test it out for you.

A couple other quick fixes you could try that may or may not work are:

  1. Restting the PRAM (see this guide)

  2. Recalibrating the display. (see this guide)

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.