Off Flavor Help Needed

I've made around 15 batches of extract homebrew and they've ranged from OK to great. Generally, the batches that I'd rate "OK" tend to have the same off-flavor, a very faint earthy to grassy taste that is most detectable in the after taste. I orginally suspected the taste was due to yeast residue in the beer. However, I'm starting to wonder if it might have something to do with hops. Most of the beers I've detected the taste in were made with Nottingham dry yeast. The most recent beer with this flavor, however, was made with S05 dry yeast. The beer styles in which I've detected this off-flavor range from Steam Beer to American Wheat to Dortmunder.

Does anyone have an idea of what might be causing this?

You dry hoppin? How long? What temps are ferments generally at? Need more info.

Nope, no dry-hopping. Fermentation temps are generally on the low end of room temp - 68 degrees F or so. The varieties of hops used have varied, but they have all been pelletized. Generally, we have done full, 5 gallon boils. Most of the batches have included bittering and flavoring/aroma hops. Also, some of the batches have used two-stage fermentation, while at least one of the suspect batches only went through single-stage fermentation.

Palmer says that grassy flavors are primarily caused by improperly stored ingredients. Old or improperly stored extract can be the culprit, or it can be your hops, or both.

If you're talking about a kind of astringent off flavor you might check the PH.... I was getting some off flavors fairly often using bottled spring water, bought some PH 5.2 to use for the steeping grains and that helped.