custom NSSliderCell

I've sort of accomplised implementing a custom slider cell that can draw over using images for the scroll bar and knob. The only obstacle that is in the way now is this, when I drag the knob quickly, the images get messed up. I've posted a screen shot.

Here is the code:

#import "customSliderCell.h"
@implementation customSliderCell
- (void)drawKnob:(NSRect)knobRect {

    NSImage * knob = knobImage;
 [[self controlView] lockFocus];
 [knob
 compositeToPoint:NSMakePoint(knobRect.origin.x,knobRect.origin.y+knobRect.size.height)
 operation:NSCompositeSourceOver];

[[self controlView] unlockFocus];
}
- (void)drawBarInside:(NSRect)rect flipped:(BOOL)flipped {
rect.size.height = 8;

    NSRect leftRect = rect;
    leftRect.origin.x=0;
    leftRect.origin.y=2;
    leftRect.size.width = knobrect.origin.x + (knobrect.size.width);
    [leftBarImage setSize:leftRect.size];
    [leftBarImage drawInRect:leftRect fromRect: NSZeroRect operation: NSCompositeSourceOver fraction:1];

    NSRect rightRect = rect;
    rightRect.origin.x=0;
    rightRect.origin.y=2;
    rightRect.origin.x = knobrect.origin.x;
    [rightBarImage setSize:rightRect.size];
    [rightBarImage drawInRect:rightRect fromRect: NSZeroRect operation: NSCompositeSourceOver fraction:1];
}

ah i'm so close. any help regarding as to why this happens and how to solve it will be greatly appreciated, thanks!