What size tank, and how heavy is your bio-load? DSB or BB?

ORP of 399 sounds on the high side to me, close to too high. I run a 200mg ORP full blast 24x7 (500g system) and ORP never goes over about 330. I have always thought 350 was an upper target, and above that you were removing too much organics.

Any chance it might be a probe calibration problem?

Maybe Gary or Todd can offer their thoughts?