The hashrate instability issue doesn't affect only CN V8 and Heavy.

It also concerns CN Fast (MSR) with the 0.33b4.
One of my RX 570 hashratr went from ~1200 to ~900 h/s.

That made me to switch to the 0.33b3 which seems to work fine so far but I yet have to compare its hashrates with SRB's.