[Sephiroth]

The 22 tap point would lower upstream power from what it was on 29dB.

Anyway, with high upstream power when the downstream is normally low indicates a difficulty being read properly by the CMTS. I'd assume nothing wrong with your modem; there may be something noisy or breaking down in the local cabinet but I doubt it. There may be noise on the optical node (tired lasers or summat). You are at 64QAM so there's less tolerance for noise. Could be noise ingress. Could be anything.

Igni usually knows the likely causes of high upstream power.