I recently changed out my sparkplugs (Bosch with 4ok on them). I put in NGK this time per IPD recomendation. The old plugs were obviously black on the tips like the car was running too rich. I put in a can of fuel system cleaner and thought that would help. The next day, the car experienced a loss of power and I traced the problem to a fuel fouled #2 sparkplug. I dried the plug, reinstalled and drove the car hard for ten miles, all seemed ok. Next day the check engine light came on but no driving problems. Today my wife reported driving problems much like another fouled plug. I am 120 miles from nearest Volvo shop and would appreciate any help keeping my car running past its current 160k.

It could be: faulty fuel pressure regulator, air mass meter, manifold pressure sensor, O2 sensor just to name a few. Read out the fault codes as a start. Steve Ringlee's 700/900 FAQs tells how to do that.