Volvo: 2000 volvo s40, turbo air flow, volvo s40

Question
I bought a volvo s40 (2000) with 111000 miles.  the check engine ligth keeps coming on and off. I changed the timing belt, fuel regulator, central control module for the lights, becuase i had problems with all those.  Now, the car runs good other than when i drive it and gas it all the way,around 20-40 mph it starts to serge back and forth until i let off the gas.  Also, when i press the sport transmission button around 70-80 mph, and gas it, it does the same thing and then the check engine light starts blinking and then turns off when i bring it back to normal.    I taken to a mechanic, and he checked the codes, one of them has to do with the turbo air flow, and i don't know the other.  I can't figure it out, please help me!  Thanks!!!

Answer
Sorin,
I would continue to follow the codes that you have. Fault trace them and see where that leads. Also drive the car with a fuel pressure gauge attached and see what the fuel pressure is when the surging occurs. This make help, especially if the pressure is not steady during the surging. Possible low pressure, clogged filter, dirty injectors.