VGA Display Problem: rapidly rolling display on the projector screen

I've got the AO756, running Windows 7 with 64-bit.Purchased in late Fall 2012.

For months I've had zero problems using a VGA cord to display presentations and video clips on our AV system. It's just been plug and play, in fact, with no need to use the FN + F5. Once in a while some message would pop up from the system tray about there being a better resolution to run, but I'd ignore it and go on my merry way.

NOW, HOWEVER, I'm getting the following: Either (a) A blank purple screen, no matter what commands I try to use to project on an external screen. Or (b): Whatever is displayed on my monitor projects onto the pulldown screen, BUT the image rolls/scrolls rapidly -- kinda like in the olden days when analog TVs displayed a "rolling picture," and you had to adjust with buttons to get the picture to slow down and finally hold still.

Any ideas? I've tried different VGA cords, different laptops (different versions of Windows), and the problem is with the Acer. What could have happened to make this change all of a sudden, like that? Thanks in advance!