I borrowed a copy of Gregory Noonan's "Brewing Lager Beer" from a friend, and in the troubleshooting section, if offered some possible new insight on this issue:
Bitter-Vegetable Taste: From deteriorated hops (oxidized B-acids).
What sort of hops are you using? How are you storing them? From your original post, is sounds like you've only had this problem in the last few months. Perhaps fresh hops are in order?