Off flavors with dark malts

I have been getting some off-flavors with my beers that include dark malts such as roasted barley. Two stouts and an Irish Red have all had this off-flavor. The off flavor is difficult to describe, sort of a really bad after taste or harshness.

I have not changed my crush and I am careful not to sparge with water that is too hot. I don't let the mash temperature go above 168-170 when sparging.

The only thing that I can think of is water chemistry. I live in Boulder, Colorado and I know that they rotate my water source between 3 sources during the course of the year. This off flavor has been present in the last couple of months. I brewed a bunch of darker beers over the summer and this flavor was not present.

I only get this flavor in my darker beers. Lighter beers (no roasted barley or chocolate malt) taste great. Is this a pH issue?

Suggestions? Thanks.

It could be that your water is very low in alkalinity and soft so the acidic malts make your mash really acidic. For giggles, you can test this by making up a really small mash of a similar grainbill and use some colorpHast test strips. The best thing to do would be to get a water report or send a sample to wardlabs

I will have to start measuring the pH of my mashes.

What flavors would I get out of a low mash pH because of the acidity of the dark malts?

Without getting into a lot of water adjustments, would Five Star's 5.2 pH stabilizer help?

This time I brewed a hoppy amber ale. I was shooting for 16 SRM. Here is my water information and adjustments. I still got the same bad taste. I do not get this taste with any of my paler beers. I also got this bad taste in a brown ale that I recently brewed. Any ideas?

Starting Water (ppm):
Ca: 4.7
Mg: 0.892
Na: 33.8
Cl: 27.8
SO4: 7
CaCO3: 40.5

Mash / Sparge Vol (gal): 5.5 / 4
Dilution Rate: 0%

Adjustments (grams) Mash / Boil Kettle:
CaCO3: 6 / 4.363636364
CaSO4: 1 / 0.727272727
CaCl2: 0 / 0
MgSO4: 3 / 2.181818182
NaHCO3: 1 / 0.727272727
NaCl: 0 / 0
HCL Acid: 0 / 0
Lactic Acid: 0 / 0

Mash Water / Total water (ppm):
Ca: 131 / 131
Mg: 14 / 14
Na: 47 / 47
Cl: 28 / 28
SO4: 90 / 90
CaCO3: 211 / 211

RA (mash only): 109 (14 to 19 SRM)
Cl to SO4 (total water): 0.31 (Very Bitter)

My mash, sparge, sanitation and everything else stays consistent between my beers. I have been adding my strike water to my MLT, most of my grain, then the brewing salts, and then the rest of the grain. After that I mix it all up before letting it rest. I add the second addition of brewing salts to the kettle and don't add anything to the sparge water.

Is the calcium carbonate (chalk) not getting dissolved? Is there a better way to add the salts?