Yes, injecto- cleaenr is your friend. Better running on gas would be becuase you likely had leaky petrol injectors messing up you metering. Did it a small amount of petrol on gas like mine did?
If one leaks you'll be getting neat petrol flooding one bore and the lambda weakening everything globally, so it'll run weak on the otehr boress. Higehr wear and lower performance and burning petrol too. More on thsi at:
http://www.volvoforums.org.uk/showthread.php?t=183595
I used a tiemr relay in the finish...
If yo have the P2 model it may be easier to put he clener in the hose, the distributro ia hardly easy to get to. But put it in via the motor if you can, via the hsoe is not so targetted. On a P1 like mine the distributor sits above the engine on the bulkhead so this an easy job.