A very strange problem with my ST205 GT4

I have experienced a very strange problem with my ST205 GT4 and thought I would share it with you all to see if any of you have any ideas.
I used to have the boost set to 18-19psi, and everything worked well. It pulled hard, and I was generally pretty happy with it.
I can't remember exactly when, though I realised a couple of years ago that I wasn't generating much boost. I did a lot of fault finding changes, including changing the manifold and turbo gaskets, fitting a different wastegate actuator, adjusting the boost controller, do a pressure test etc, and it still wouldn't generate more than 13psi of boost.
I even disconnected the wastegate actuator completely, and it still would only run up to 13psi. There were definitely no leaks, as I used a proper leak tester.
A couple of weeks ago, I drove the car, and gave it a good squirt, and the boost controller flashed red, which means that it got up to the boost warning limit that is set, which is 20psi. I thought "what the fcuk?!"
The car is now boosting consistently to 18-19psi without me making any changes at all.
The only thing that I can think of is that the wastegate flapper wasn't closing properly before, and now it is. 
Do any of you have any other theories?