I agree. The "monitor" or BordMonitor as it's called (the screen) won't have anything to do with your problem if you're experiencing these symptoms. It's the GPS CD/DVD unit in the boot. Your dealer should know the difference.

The screen will flicker if the system is in a reboot cycle, but the dealer should be able to ascertain if it's still flickering after the system has correctly booted. They're usually (but not always) right.

It's about £3500 for a complete Nav system so you probably won't want to do that. However, you can pickup new computers (the CD/DVD) unit in the boot for about £250 on ebay quite readily. You can possibly also replace it with a unit from a Rover 75 for around £75, but I've never tried that and I wouldn't know the right part number.

I replaced my previous Mk3 (CD version) with an upgraded Mk4 (DVD) version when it started exhibiting this problem. Many months later, the new one now does the same thing which makes me wonder if this is a much more common problem with these units than is widely understood.

Replacing your unit with a newer one should fix the issue - at least for a while, providing the one you get doesn't already have the issue.