Overflow level fluctuating, why?

[input80]

So, somewhat new 210 (I think) gallon custom tank 72*30*24. Been running since about Feb. New DCT-15000 return pump, external overflow, herbie drain.
Since day 1 the overflow level fluctuates & I can't keep it the same for more than a day; with minimal adjusting on the gate valve. I once had a Waveline pump cause this & changed it out, problem was solved. I changed out the pump only yesterday & still fluctuating. I have removed & cleaned out the gate valve a few weeks ago, there was really no build up in there. So my question is, could the Spears (bought brand new) gate valve be the problem, the return pump controller or power supply. I will change out the controller & power supply this weekend, incase there is something faulty there.
Any ideas where else the problem could be? Could I have missed any pertinent information? 1.5" drain & 1" dual return lines; I have the return pump running at about 7 out of 10 on the controller.