Check your turbo to intake manifold hoses. The telltale signs are malfunctions only under boost conditions and horrible fuel consumption. There five in all: the short blue one at the turbo, the long curved one from the elbow tube to the intercooler, the short coupling from the intercooler outlet to the straight pipe, the special shaped one from the straight pipe to the intake manifold, and the smaller one from the straight pipe tee to the idle control valve. All are important and should be checked inside and out for holes or deformations. On a high mileage turbo oil leakage will eventually cause deteriorations. A small hole can cause problems all out of proportion to its size; never underestimate its effect, never overestimate what you think duct tape patching can fix. The good news is that mechanically the fix is rather easy and quickly done by a novice if the engine is cold, the bad news is that the hoses are only available from your Volvo at I pay dearly (ipd) prices. But bite the bullet and replace them. If your boost has been increased over the stock setting good hoses are even more important.