Sour astringent taste in Belgians

I started doing partial mashes and all grains brews this year. Probably done about 8 - 10 batches. Most have turned out great. Two of them though have a similar sour, astringent taste. Not totally undrinkable, but definitely an undesirable taste. Something that stays in your mouth for a long time. I have been experimenting a bit, so the mash temps have varied, but definitely well below 170. Technique is using a BIAB in a cooler. One of the affected batches was dunk sparged with 170 deg water, the other i just slowly poured a gallon of 170 deg water over the bag (I often do this). One was all grain, one was PM with a can of pilsner LME. I use tap water for the mash. I have no idea what's causing this. Two things that come up as sources of astringency are water ph, and high mash or sparge temps. I don't think its water because it's only happened with these two batches, and is not at all present in the others. I don't think it's mash temps either since I struggle to keep it between 150 and 155, and i suspect the cooler is warmer near the top of the mash which is largely where the thermometer sits. Racking my brain for what's common about these two batches. Only thing I can think of is both had Belgian Pilsner malt as the base. Anyone have thoughts about what might be causing this?

Could you describe the flavors a bit more? http://www.howtobrew.com/section4/chapter21-2.html

Based on your descriptions so far (a lingering bitter-sour), I think of phenols. These phenols come from the malts and hops (yeast produced phenols tend to be peppery or spicy). The undesirable flavors imparted by these compounds can be enhanced by the minerals in your water. Any idea what your calcium and sulfate levels are like?

Phenols (and some of the associated astringency) do drop out over time. If you haven't already, consider cold crashing (causes coagulation of phenols and proteins, allowing them to drop out) and a fining agent (I like gelatin in a cold keg).

Here's some details from Arlington County.

Hardness: 7.5 grains/gal or 128 mg/L

pH: 7.7

Chloramine Residual: 2.8 ppm

Fluoride: 0.7 ppm

Sodium: 22 ppm

Nickel: 2.1 ppb

Calcium: 39 mg/L

Chloride: 32 mg/L

Magnesium: 7 mg/L

Sulfate: 47 mg/L

Do you treat your water? Add/remove minerals? Remove chloramine?

Welcome to the forum, by the way.