"UNDO" & "BACKWARD" not working for brush strokes?

Question

After making three consecutive brush strokes in my image:
Click on Undo once, History list backs up one step, but last stroke remains on screen. Press Undo again (Redo) and History list returns to last position, last stroke is still on screen. Click Undo again, History list backs up one step, but now brush stroke disappears as it should.
Or, after clicking Undo the first time, the brush stroke will disappear if I apply another stroke or enter a command, e.g. add new layer.
After making three consecutive brush strokes:
Use Backwards command, the History list backs up one step, but brush stroke remains on screen. Use Backwards command a second time, History list backs up another step, the last two brush strokes disappear from screen. Use Backwards a third time, History list backs up another step (now 3 steps total), and the first brush stroke disappears as it should.
These commands work fine when creating three consecutive layers, then undoing or moving backwards.
I am using CS 5.1 for Mac running OS Lion 10.7.4. This problem occurs using either a trackball, Wacom Intuos4 tablet or keyboard shortcuts. Can this be a screen-draw bug in the brush tool? Thanks.

Answer 1

*Rich* wrote:
I still don't know what Vertical Sync pertains to relative to OpenGL Settings. Anyone know? Thanks.
Only Adobe knows the exact implementation under the covers, but what it conceptually does is prevent display updates in the middle of a scan. 
Keep in mind that what you see on the screen is sent pixel by pixel, row by row, to the monitor - much as it was done in the time of CRT displays and scanning electron beams, even though today most  have LCDs.
Once the last pixel at the bottom has been sent, there is a time called the "vertical sync" time where nothing is sent, before the "scan" starts again from the top.
Setting the "Vertical Sync" option is supposed to limit display updates to only occur only during the vertical sync time.
As Conroy has said, this generally can make a display seem cleaner, as when moving something around you might see part of it moved more than another part without Vertical Sync.  This is called "tearing" (as in "tearing a piece of paper in two and having the two parts not line up").
By definition the Vertical Sync option setting causes a difference in the timing and the sequence of display operations, and that may be working around a bug in the display driver on your system.  It sounds kind of far-fetched, but it happens more than you might think.  Adobe provides all these options to help work around problems.
-Noel