If it's powering up ok, then rebooting etc.., it sounds VERY much like the internal hard-drive has developed a fault.
I have a RT4 in my Pug 407 and the exact same fault occured on this, simple solution - take head unit out, take cover off - remove CD assembly/cabling (be very careful not to break the ribbon cable), then remove the hard-drive.
Try and source an identical model, a normal computer/laptop hard drive won't suffice I'm afraid due to the shock/temperature differentials which it will need to endure, the hard drives placed in cars are more 'industrial', and can handle the shock/temperature changes.
If you can make an image of the faulty hard drive, do that as it will save you some time reformatting your new hard-drive, I personally use the latest version of Norton Ghost (v15 I think), but I'm sure there is plenty of other software which could do this for you.
If you are successful in making in image of your original HDD, the put that image on to your replacement HDD.
Lastly, refit replacement HDD and test.
If you couldn't clone/backup your original HDD you will have to install the operating system from an installation CD, of which can be sought from various places on the web :thumb:
Might save you alot of money, as a replacement HDD should only cost you between ÂŁ30-50 I suspect, as the capacity of the HDD's Pug use are not very big, ie: 30/60gb.
Best of luck :thumb: