I own a 92 jdm Rx7 FD which is totally stock except for a hi-flow cat and some suspension upgrades.
150,000km or (90,000miles) with 30 - 40k on rebuilt motor.
The car has always run great never any major problems, until the other day.
I took the car out for a spin on a warm day and its the first time i goosed her (WOT) in a few months.
In 2nd gear full boost everything seemed great untill i hit 5000rpm then suddenly the motor shut down??? Once the revs came down everything worked again?? Scared the crap out of me! Now it is a consistant thing, however if i really push it i can get it past 5000 but boost creeps to about 12psi.

In addition when ever im coasting with the revs below 3000 in 3rd, 4th, 5th gears it does the same thing, this is like a violent bucking as the moter sputters on and off.

Ive read alot of post about sputtering, grounding, and the ecu but nobody has a solid answer for this.
Does anyone know what im talking about? is there an answer?

 

Most likely, you are hitting fuel cut. You either need to upgrade the ECU or get a boost controller to limit your boost.

 

Sounds like ignition breakup or fuel cut. (More likely ignition breakup I would think.)
As its ran fine in the past, I'd check your plugs to see if they're okay. Could also be coil, or the leads to your plugs.
But as Mahjik states, you either need a more restrictive exhaust, or a different ECU ideally. Overboosting is a sure way to pop motors.

 

I think Mahjik nailed it. It's cold now, so boost creep and fuel cut is probably the cause. The stock boost "control" relies on a pre-programmed map and does not use actual boost levels to control boost. Your high-flow cat may be enough flow improvement in this cold weather to hit fuel cut. See the chart below:

 

2 things might be
vaccum hoses or spark plugs?
again thats just me