As someone who has an ATI x1400 card...I can almost guarantee you your problem stems from Compiz.
To check if compiz is the problem, go to the mintmenu > Preferences > Appearance > Visual Effects
And set the level to "none". As of right now the ATI drivers for Linux available aren't QUITE as up-to-par with Nvidias, and there are some problems that can arise if Compiz is enabled. These including OpenGL applications, games, screensavers flickering.
To boot, the drivers that Ubuntu and Linux mint currently have are outdated (with luck an update rolls around soon enough) or in some cases just broken.
Current drivers available straight from ATI: 8.9 (Haven't been able to review. I'm a tad lazy to try their installer. ^.^;)
Drivers in "Hardware Drivers": 8.3 (Stable driver, but lacks XV video output. Might want to use OpenGL output with movie players that support it)
Drivers in Envy: 8.6 (I do NOT recommend this driver for now.... The driver is completely broken with cards such as the mobility x300 and x1000 series cards. (at least my x1400, and I have heard of, and unfortunately experienced, things such as complete corruption of the screen display until reboot when trying to use a game through wine (game previously worked), and getting stuck with full screen youtube (Well...more like an overlay that's jammed on than youtube being stuck enlarged. You can recover from this problem, but you need to guess where the firefox window is and close it)))