When a turbo feels like it is going on and off that is called Compressor Surge or sometimes just Turbo Surge (the first is more correct). That happens because you are operating on the left hand boundary line of a Turbo Map. It is an area of massive instability in the in the pressure on the compressor wheel. And why you see it in 5th gear is probably because the cars throttle is too closed off so you are getting back pressure in the intake manifold causing an instant build up of extra air in the process-- this creates areas of high pressure and low pressure-- this causes the compressor wheel to create boost and then stop creating boost. I know this engine isn't a throttle body type system but it still has to control the air flow into the cylinder heads some way. It seems that is not working correctly. So, why does vanish in 4th ? The throttle is opening up more in 4th Gear to allow more fuel and air to mix and this gives the turbo the ability to pump all that air into the cylinder head. Why this didn't show up right away is sort of a mystery. It could be that your tunings some how reset to stock and you need to reset them. But I don't know much about your settings so those claims are hard to make by some one like myself. I would just take it to a mechanic have him test it out and see what he says. I bet Compressor Surge is what you are describing and it will cause the cylinder heads to trap excess exhaust cause if it is by an intake blockage. It could be a valve timing issues too-- the VVT system is some how cutting off the flow from the intake too early and causing the surge. Compressor surge is not good and you should have it corrected as quickly as possible.
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