FEATURE FOR DIGIBYTE
You can set static diff by setting sd=XXX text at miner's password.
e.g.) sd=8
set
ccminer+ -r 0 -a myr-gr -o stratum+tcp://hub.miningpoolhub.com:12005 -u ***.* -p sd=1
on pool Difficulty 4.01

how to fix?
Oh it doesn't work with 12005 port. It works only for direct ports currently. 20499 is Digibyte-groestl direct port.
I changed some properties for vardiff. It will not go up and down quickly like before.
I believe current new vardiff logic will work much better that static diff would not be needed.