I've never seen the hash rate stay stable on PhoenixMiner.
I've been trying it for about a month at least and the hash rates moves up and down always.
I don't know if that's common for everyone, but always for me.

An example for me is a GPU fluxes between 29.5 to 30.2 back and forth.
All my GPU's do that to some degree.

Maybe the Dev's at Phoenix can address that phenomenon to let us know if it's an issue or why it does that.
Doesn't seem to effect mining or shares but is interesting on why.