I recently decided to switch to platinum plugs for my Volvo 780 turbo. When I replaced the old OEM plugs, which looked as though they hadnt been touched for some time (I bought this car used a little less than a year ago), I found that the engine was actually idling at LOWER rpms, and the tach was sort of going up and down- the idle isnt rough, its just moving to higher and lower speeds ever so slightly (a change of perhaps, 25-50 rpm maximum difference). I recently did a pretty significant amount of work in the throttle area, and as far as I know that whole assembly has been properly serviced. I noticed the old plugs looked relatively ashy compared to what I am used to seeing in cars, and I gained the distinct impression they smelled faintly like gasoline. Could I have a bad 02 sensor? Is there a cost effective way to test and correct this?