-i 1 seems to solve the hang issue for me...
For me -i 1 solves the problem too, but speed is less then -i 5 with v12.0. -i 2 gives the same speed as -i 5 in 12.0 but hangs often...
sadly after 3hours got a hang with -i 1 in 12.1....using 12.0 with -i 2 it is totally stable...i'm using agressive undervolting with my 7990s and 295...