There really could be a lot of possible things going on. You'll have to start with the basics and move up.

First of all, check ALL the vacuum lines. Consider replacing them all if they look old. It is not hard to do, and not expensive. Replace tee fittings as well. Check also the flexible hoses from the turbo to the IC and then to the intake. Really feel them to see if they have soft spots or holes.

When you drive, can you hear anything that sounds like a boost leak?

It could be lots of things, like maybe you aren't getting enough fuel. Do you have any pinging? Maybe your cat is clogged. Maybe your wastegate is stuck open a little. Is the engine cutting out at all when you hit boost? Does the car feel quick at 5 psi? What RPM do you hit 5 psi at, if you are taking off from a standstill?

The G-valve probably wouldn't solve your problem. The G-valve is definitely a great mod, but get your engine running properly first.

Greg