Check the coolant temp sensor (Volvo part number 9186486). I had the same problem with my S70 (same sensor), where it was reading a constant temp of 100 degrees celsius when it was actually 10 degrees fahrenheit, causing it to deliver too little fuel (no "check engine" light was lit). You also might want to check to see that the shop that performed the maintenance used genuine Volvo plugs, as some aftermarket plugs just don't do the job.