Simply Koi and More Koi

I know the specs say it can be dry mounted outside the pond and I have been assuming mine has been cutting out because I am getting an airlock on my skimmer line and it has been overheating.

But now I am not so sure I am getting an airlock, because when I leave it for a couple of hours to cool down and then switch it on again, it runs fine and there is no air lock.

Anyone else used one of these dry?

Thanks, Martin