Yes it is Yahoo - with PHP it is realtively simple, if there is a bug, it fails, if not, it works. So given it works "sometimes" the only possible explanation is Yahoo - and I know they are very unrealible, probably the second worst option after WU
Originally Yahoo was used for the interactive banner at the top, i then had to change this to use METAR, which is not so region-specific, but reliable. I guess there is not much we can do or expect given it is a free API. I even had to remove the pressure completely not long ago, because there was a bug, they reported nonsense, people told them about it on their forums and they didnt do anything since March, so I deleted it.