Here is what I see.

I get hardly any boost in 1st gear.

In general I get 9-8-10 and then it falls off right after hitting 10 down to around 6.

It falls off faster in lower gears.

In 2nd gear it falls off immediatly after hitting 10, and if falls quickly to around 4.

In 3rd it hold 10 for a bit then falls not as fast as in 2nd, to about 5.

In 4th it holds 10 for quite a while and then slowly falls to about 7 by redline. I haven't redlined it in 5th lately yo see that boost pattern :p

I've already:

1) Replaced 1 split vaccume hose
2) Checked and resecured all intercooler piping.

Any other ideas?

 

That almost sounds like the rubber coupling on top of the Y-pipe. It will expand when it reaches 10psi, a possibly split on it will open up, release pressure, then it closes back up, pressure builds back up in it, and then it releases it again.

Have you checked that rubber coupling?

If that's not it, then you may want to start pressure testing the wategate and precontrol actuators and their solenoids. Assuming, of course, that you have already checked all your hoses.

 

I'm not sure how to go about that second step that you propose, but I'll do some reding and figure it out.

I'm probably going to get an Efini y-pipe to eliminate part of the possible issues.

Thanks