I just got my car running, so I have no idea how using the carb spring on the wastegate should (feel) work like.
Boost starts climbing at around 3k and at around 4k stops climbing (8psi) but then dips down to maybe 5psi. This has a unique, but not so thrilling feel to it.

My setup is a B18 with a T3 .42/.48 A/R
Is it the small turbo ? or something to do with the Spring??

 

Thats the shittyness of the spring setup, the boost will bounce a good 2-3 psi. It's hard to dial those springs in. You can try a different one, or a helper spring on it. Best way to go is just use a internal wastegate actuator. You can get pretty creative mounting those things on either your block or compressor housing.

 

mine does the same thing

 

Yea... My car was running 5psi with a single spring, so I added another one. Then it ran 9psi. The next day is was running 6psi...

I think the heat gets to the springs and makes them weaker, mine are just springs form Ace Hardware.