compressor problems

I have the dc5000 compressor and when it is not hooked to the leader hose it runs like a champ. But as soon as you put pressure on it it bogs down and barely runs. I have rebuilt the compressor part but that didn't help. The car also has a good alternator and an optima red top so power is not a problem. I have had it for over a year and it just now started doing it.

Thanks for any suggestions.