Help any ideas?!!!

[Iloveadrianmole]

I have a four year old Chihuahua. He's quite a sturdy, confident little thing - well trained (during the day!).

At least once a week he starts yapping in the early hours of the morning and this will then go on all night until morning.

He will bark/yap loudly once and pause for about a minute then bark again then pause and so on all night.

He is well fed, has plenty of water, two good walks a day and the run of the house and garden (he has his own dog flap) and plenty of human attention - I am at home all day with him. He sleeps in a very large dog crate has a good cushion and a blanket to burrow under (Chihuahuas like burrowing) and is not in a cold draughty spot. He can go out if he wants the loo in the night but never does. We have tried ignoring him and not giving him attention, making him go out for a wee and going to see what is wrong. When we do go down he just looks at us as of to say "what's wrong with you?" And doesn't come out of his crate - but as soon as we go back to bed he starts again!

Any ideas what could be the issue or indeed how to stop it!?