Your rpms can still increase as it's making power, the boost is fluctuating from the air stalling (surge). If you could do it at WOT in a taller gear you would get to a point were the engine could use the air and the surge would stop, if it didn't kill the turbo.
I'm assuming it's a V or H trim is it's a T04B. If it's making that much boost at 30% TP with a V trim at that low of an rpm, it's compressor surge. The cheapest way to slow it down would be to go larger on the hotside a/r, like a .82. Going with a stage 5 wheel would work too but you would need to machine the turbine housing for it.
I ran a full TO4B/ V trim compressor, O trim turbine (bigger than a stg 3) .69 a/r (alot bigger than a T3 .63) and could get it to surge at low rpms, part throttle.