I had this with a number of my early extract batches. All were also brewed in a stainless pot with store-bought spring water, so that wasn't the issue. These were fermented in glass and put into secondary, then bottled. Only no-rinse sanitizers were used. I don't know that the causes listed in "metallic" quite fit, but to me it tasted metallic. So perhaps we're talking about the same thing.
Possible things:
1) Stale/old extract. (Is this the "extract twang" everyone talks about?)
2) Oxidation. (I could see the "sherry" description being similar here)
3) High pitching/ferment temps, underpitching yeast (I think some of the higher fusels might give this flavor and/or yeast reproduction byproducts)
I wish I could tell you exactly what made it go away. Batch #3 didn't have it. I switched to all-grain by about batch #7 and it was mostly going away by then. The early batches used an autosiphon that kept losing prime, so oxidation could have been an issue on those batches (especially since I went from primary to secondary to bottling bucket, so there were two racking steps that could introduce O2). Around that time was when I was making rudimentary starters (i.e. boiling wort & pitching yeast the morning before brewday started or at best the night before). It was around then that I learned that you don't want to pitch your yeast into 80 degree wort too. So a LOT of brew steps were changing all at once.
The only thing I'd say is from an ingredients perspective, make sure you're buying ingredients from a source that will be sure to have fresh extract. And potentially a no-rinse sanitizer like star-san that is acid-based might be better than something oxygen-based like one step.
Other than that, look at process. Practice proper racking technique several times with water to ensure that you're not introducing bubbles of oxygen during the process. Try starters (or dry yeast) to ensure you have proper cell count and watch pitching and ferment temperatures.
Hope this helps... I think I had the same off flavor, and these were the improvements I made over those first six months that mostly made those flavors go away. I just don't know which one was the critical step.