I think your on the mark Chris.
That new pipework for the wastegate hose is pretty long, so this may be why your boost is creeping up.
Also, if it's not the right type of hose, it can buldge under pressure too which won't help things.
One thing to try, and that's to unplug the factory solinoid (electrical plug), and see what that does. If it fixes the problem then you know it's a problem in the pipework from the factory solinoid - but I dought it'll be that.
The only other thing to try is to take the factory solinoid out of the circuit totally, and just run a pipe from the wastegate to the inlet.
J