Sounds like seperation anxiety to me too, we had simular problems with Milo a while back, we followed a simular treatment method as mentioned on that link and we can now leave him for 4-5 hours without problems.

I took this from the link as it seemed to be particularly effective for us:

Departure and return should be made as quiet and uneventful as possible to avoid overstimulating the dog. The dog should not be given attention prior to departures nor given attention and praise upon returns. Excessive attention prior to departure and upon return seem to increase the anxiety during separation and it does NOT make it easier on the dog as most people suspect.