I have the identical car as you, albeit a petrol version, but the radio will be the same. The only time my radio will go into the 1 hour mode is if I am just sitting in the car without the ignition switched on. If I hit the volume control, which is also the on/off switch, then it will go into one hour mode. I've never know the radio to go into 1 hour mode any other time.

You say you've already changed the radio itself, which would have been my first thought. The only other possibility I can think of is a fault in the ignition switch, but, if this is the case, then it must be intermittent.

Sorry, I can't offer any further help.