hi mate, iv got a 4x4 saph and mine did somthing simular, when mine just came onto boost at around 3-4k rpm it poped and banged, blew flames out an missfired, as it did this the rev counter went through the roof.
i changed all possible sensors but still didnt cure,in the end it was dodgy connections on the 3 wire multiplugs on the engine loom. they are on the bulk head just above the rocker cover.
try puttin gresae in their first but i actually cut mine off and soldered wires together, no problems since
hope that helps