It might be the yeast...I would check sanitation practices...try to find some rhyme...or reason for the sour...in one year it may be a poem...
I recently had a beer turn out so sour I thought it had been infected. Erockrph suggested dosing a glass with a little baking soda to see if it was just too low on pH. It worked!
Its cheap, it's easy, give it a shot.