rynberg

Doubtful. You most likely have a loose or split vacuum line, or blown check valve, or similar. Read the FAQ, get out your FSM, read this link - http://www.autosportracetech.com/RX-7/rx7stuff.htm, and get your hands dirty.

VegasFD

Don't worry about the gaskets. Check all your IC couplers starting with the one on the y-pipe, and work your way to the TB. The one on the Y-pipe/crossover pipe is proned to cracking, so take it off and inspect it thoroughly.

Edit: Oops, should have read the question better.

dubulup

if you are using stock IC hoses...I bet the lower one is split in half under the "retension rings"

Liem

Agreed. I blew two of those within a month. The result was 4-5 psi on both turbos with slow transition. If left that way for a couple of days, the boost may even drop to 2-3 psi.

White94RX

Yeah, sounds like a boost leak. Most likely culprits are the couplers, like the one at the Y-pipe.