Canvas.drawBitmap() is intermittently slowed, causing white flashes

I am working on a live wallpaper with a scrolling background. I have two bitmap objects which I alternate between in order to keep the previously drawn pixels for the next frame. I draw a new line at the top of the canvas, then call drawBitmap to copy the rest of the pixels onto the canvas.

I am using a Runnable object to do the heavy lifting. It does all copying and calculations required and then locks the canvas, enters a synchronous block on the holder, and makes a single call to Canvas.drawBitmap(bitmap,rect,rect,paint). Occasionally there will be a white flash on the screen, which seems to correlate with high CPU activity. In using traceview, I found that the drawBitmap operation, specifically Canvas.native_drawBitmap(), is taking much longer than normal. Typically it completes in 2-4msec, but when I see a white flash, it can take anywhere from 10 to 100 msec.

private void draw() {
    SurfaceHolder holder = getSurfaceHolder();

    Canvas canvas = null;
    prepareFrame();
    try {
        canvas = holder.lockCanvas();
        synchronized (holder) {
            if (canvas != null) {
                drawFrame(canvas);
            }
        }
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        if (canvas != null)
            holder.unlockCanvasAndPost(canvas);
    }
    afterDrawFrame();
    handler.removeCallbacks(drawRunner);
    if (visible) {
        handler.post(drawRunner);
    }
}

The draw() function is called in the run() of the Runnable.

private void prepareFrame() {
    num++;
    if (num%2 == 0) {
        mainBmp = mainBmp1;
        mainCan.setBitmap(mainBmp1);
        mainCan.drawBitmap(mainBmp2, source, destination, null);
    } else {
        mainBmp = mainBmp2;
        mainCan.setBitmap(mainBmp2);
        mainCan.drawBitmap(mainBmp1, source, destination, null);
    }
}

The prepareFrame() function is h