Strange monitor glyph problem after upgrading to Mint 11

[a coder]

I previously ran Ubuntu 10 on my work laptop (a Dell Precision) with an external 21" Dell monitor. I use the Zend Studio IDE for development, and never experienced any display issues in working with that application.

A few months ago I upgraded from Ubuntu to Mint 11 (Gnome 2.32.1). All hardware works including wireless and such, however I now have a new problem when working with same version of Zend Studio. Frequently and randomly, my code ghosts itself, as shown in the image attached. I usually make the glyphs go away by scrolling my mouse wheel a little or clicking somewhere on the screen. The only variable here is changing from Ubuntu to Mint. My hardware setup is unchanged.

What could cause this?

[a coder]

Just logged into Gnome Safe Mode and the glyph issue went away.

Any ideas?