The stock boost solenoid, in my experience, worked fine at this level. Make sure you've got proper fueling first, then look into a HD adjustable actuator if you plan to turn it up.
My guess is that you've got the RJC controller set to around 10#'s, so when you reach that psi, it's pulsing the boost signal to the old stock actuator that is opening the wastegate at somewhere less than the original 15lbs, causing the fluctuation you see. Then when you go WOT the boost goes straight through the RJC at 10#'s, allowing the full signal to been seen at the actuator, fully opening the WG and limiting max boost to ~15lbs by design.
You should be able to test this by first eliminating the RJC controller & verifying the stock actuator. If this checks out & you have no KR, you should re-install the controller & turn the adjustment in gradually to achieve your desired boost (approx 1 full turn per #), up to the actuator's verified limit. The RJC controller will then allow you to turn the boost up beyond the actuator's verified pressure to achieve higher boost, up to ~23#'s, which is now limited by the stock turbo.
If you do not intend to exceed the given WG actuator's limiting pressure, then you do not need the controller.
Hope this makes sense.