quicktime mpeg playback speckled

Not sure how to best describe this.

I'm a recent switcher to Mac and am using a 20" iMac G5. I've been watching roller coaster videos that are in MPEG-1 format from www.themeparkreview.com, for a while, previously on my PC using the ATI file player or WMP, now on the Mac with Quicktime Pro.

It seems to me, on the Mac, the videos look worse then they did on the PC. Specifically, the backgrounds have a "speckled" look to them. Sort of like old scanned 256 color images did when they had to use dithering. It's not that bad, but it is annoying, and looks worse than the PC did. I was using a 19" monitor, and since the videos aren't widescreen, the displayed images are almost identical in size (so it's not just a bigger monitor resulting in more noticable pixels).

I haven't noticed anywhere to change the color depth in OSX as you would on a PC (i.e. 16-bit / 24-bit color). Does anyone have any suggestions? Everything else looks so much better on the Mac, but these particular videos don't!
They play good on my Mac. Maybe if you list your Mac specifications someone here can help you.
Thanks for the reply...

I should clarify a bit that I'm being really picky here. They play "fine" on my Mac too, when looking at what should be a solid color or at least a smooth gradient, especially in motionless areas (e.g. the sky), you can see pixels that look to be more dithered than you'd expect in higher than 16-bit color.

Does Quicktime use hardware MPEG accelleration? I assume the ATI video cards support it (their PC equivalents do, anyway).

Is there another (free/cheap) mpeg player I could try to compare quicktime with?

I'm using a 2.0GHz iMac with a 20" display, Tiger 10.4.1, Quicktime 7, and FWIW, Quartz 2D Extreme is enabled (although disabling it makes no difference).