I'm having a very specific "bug" in my iPhone application. I'm setting two images for the highlighted and normal states of a button. It works as expected when you "press" and then "touch up" at a slow pace, but if you click/tap it quickly, there's a noticeable flicker between states. Is this a known bug or am I setting the states incorrectly?

Here's the code that creates the buttons:

UIImage *normalImage = [[UIImage imageNamed:@"btn-small.png"] stretchableImageWithLeftCapWidth:10.0f topCapHeight:0.0f];
UIImage *highlightedImage = [[UIImage imageNamed:@"btn-small-down.png"] stretchableImageWithLeftCapWidth:10.0f topCapHeight:0.0f];

[self setBackgroundColor:[UIColor clearColor]];

[self setBackgroundImage:normalImage forState:UIControlStateNormal];
[self setBackgroundImage:highlightedImage forState:UIControlStateDisabled];
[self setBackgroundImage:highlightedImage forState:UIControlStateHighlighted];

[self setAdjustsImageWhenDisabled:FALSE];
[self setAdjustsImageWhenHighlighted:FALSE];

When a button is tapped it simply disables itself and enables the other button:

- (IBAction)aboutButtonTouched:(id)sender
{
    aboutButton.enabled = FALSE;
    rulesButton.enabled = TRUE;
}

- (IBAction)rulesButtonTouched:(id)sender
{
    rulesButton.enabled = FALSE;
    aboutButton.enabled = TRUE;
}

Any thoughts on this quick-click flicker?