I am amazed that just gentle walking can raise your heart beat so much.

Have you checked your blood pressure when you hit 90 bpm?

Do you take any aspirin or ibuprofen? Both are blood thinners and can make it so blood flows through the brain easier. Even low dose aspirin may be worthwhile. Enteric aspirin does not dissolve in the stomach so it is less upsetting.

I take 2 975 mg doses of enteric aspirin each day for arthritis (am and pm). It also keep my blood very thin.