1) find out all of the mods that have been done to the car.
2) get an aftermarket boost gauge and make sure you are not overboosting.
you can get one for very cheap here;
http://www.egauges.com/vdo_mult.asp?Type=Turbo_Boost&Series=Vision&Cart=
the 0-30 In.Hg. to 0-25 PSI for $30 is the most popular one.
any troubleshooting done without knowing exactly what mods are on the car is pointless and dangerous.