as for the atman fixture, the only fault lies on the RX7 connector. After sometime of usage (6mths or so), carbon will build up on the connector which will prevent it to have a good contact with the MH bulb. and also, since the RX7 connector comes in 2pcs/set, after sometimes it will get loose( the mh bulb will be loose after installation).
i think you problem lies to the poor contact of the holder n bulb since it is able to light up the first time but fails after a few minutes.
My suggestion is to unscrew the holder, try moving it closer, hoping that it will give a good contact of the bulb n holder.
With that, it should resolve your problem. If not, replace for a better german RX7 holder....
my 2 cents.....