Can you post which exact map you are using? Maybe download a new one in case something got messed up/corrupted/mislabeled?
The tune should target a certain boost at a certain rpm (you can look at the 'boost target table' if you downloaded ATR). Looking at a S1 tune, the boost limits have throttle close at 17.5psi and fuel cut at 18.5psi by the look of it. Looks like you might be getting fuel cut when you hit over 18.5psi? Someone with more ATR knowledge then me can probably explain how these limits interact.
If you are consistently over boosting then there is probably something mechanically wrong that you need to find and fix, rather than a problem with the tune. Try RedDjinns suggestion first, boost leaks suck.
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