Originally Posted by Fabrik8

a PCV valve won't affect mileage that much, they're a small improvement and are mostly for emissions. This sounds like something like a O2 sensor, MAP/MAF sensor, or TPS sensor, it's definately something that is throwing the ECU fa curveball.. I don't know crap about Volvos though, so I don't have any insight on which one it could be. Does it smooth out after it warms up, or does it still run rough, just not as bad?

I'd be interested to know if your new fuel pump is flowing a lot more than your old one and the pressure isn't getting regulated properly. If it's only been doing this since the filter and pump replacement, I'd look there first. If there is a convenient place on your fuel rail to tap a fuel pressure gauge that may tell you a lot.