my car was doin the same thing, ended up being 4 pistons with detonated ring lands.... bad bad bad...
fuel cut wasnt workn, and the newly fitted gcg high flowed turbo was over boosting something severe... 3rd gear pinging burnout= dead motor..
do a compression test, and if the cylinders are uneven, or way below 160-170psi (recommended compression) you have probs!!!!