id check/change your HT leads, distributor cap, and rotor arm. also check the HT coil low tension connections for corrosion. the leads can be bought for about half volvo price, for exactly the same set/brand, from
http://www.partsforvolvosonline.com/, and the distributor cap and rotor arm can be bought from a Bosch agent for about half volvo price also, and there exactly the same parts. i use denso iridium spark plugs, but there not cheap, then again you shouldnt need to replace them for years.
p.s. one last check to carry out is the boost pressure, BSR do a good guage, if its too high get the actuator/rubber pipes checked, if its too low check the whole intake system from the turbo onwards for air leaks. it could even be a faulty air flow meter. actually get volvo to read the codes out, it might save some time and money replacing parts on the car.