I had the same thing happen with me and my blueberry wheat beer I made. I made it at the same time as a batch of blueberry wine which at that point was the same way.
Here's what happened with me, and I'll let you derive if it is applicable to you or not. For the first 2 months the beer (and wine) had a "farty" flavor/smell to it. It wasn't overpowering, but definately took away from the beer. I talked to my homebrew store and he mentioned that fruit (some more than others) can create a sulfur compound that will make an odd flavor in your beverage (as it was in mine). This usually goes away with aging wines, but since beer doesn't age as long you need to let it vent.
I plugged an unhosed connector to the gas port to let it vent for 3 weeks (it was previously carbonated and chilled at 35 Degrees F). Afterwards it tasted great. the wine (which is still aging) has lost most of it's "fartiness" but has aged for 7 months at this point, where as the beer was only 6 weeks after fermentation when it had cleaned up.
Just leave it in your fridge with a un-hosed gas connector, let it vent for 2 weeks flushing the airspace with C02 twice a week. You should notice a drastic difference. Other than that all I can think of is unclean kegging, but I don't think that's your issue.