Hi ,
looks like something is trying to regulate it .
Try to connect the wastegate actuator straight to the inlet of the turbo without the amal valve then see if it stays stable. Only moving one hose over worth a try.
Make sure you are not running to much boost by taking the amal out .
Other thing I think can happne is that if you got a 2 bar map sensor and you are going over slightly the boost it can take I am not sure what that does.
But I put a stage 1 in a friend of mine overboosting 1,2bar and holding 1 bar with all the standard equipment may be worht a try to regulate the boost 0.1 bar down and see if it is still present then you know you are at the border of a reading from a sensor.
gr,steven