Decided to get my car mot'd today as it's been sitting for 8 months now & this is perfect weather for it!

It's a 93 wrx import, got a full de-cat, apexi ecu, front mount cooler, td06 turbo (I think can't remember) otherwise pretty much standard.

Problem is when it comes on
boost around 3k it backs off
Like it's over boosting or lacking fuel?

I'll be honest I don't know nothing about these so I won't pretend to, I bought it on a whim last year as it was a nice car for money but need to sell it now. It passed it's Mot today no problems, so just that to sort on it now?

Thanks