The same thing happened to me last year. The same suggestions you mention were suggested to me. However, the culprit was a rock (or several) that passed vinegar testing.
I later found out that vinegar sometimes may not be acidic enough cause a rock to bubble.
I took all the rocks out, problem solved.
My pH is 7.8 from the tap and I was climbing into the mid 8s. You say your pH is 8.2 after a week. A swing of .6 pH over a week, in my opinion, is not too bad for everyday normal fish. You did not specifically say you tested your pH after a water change and notice this change.This may not make your fish happy, and may kill some.
Figure out which item in your tank is causing it, substrate, ornaments, rocks, or whatever, and get it out.