Di Cassette replacement... I have a 2000 Saab 9-3 SE. My check engine light has been coming on, stays on a couple of days and then goes off again. One day it felt like it was misfiring, and had a loss of power. I took it to a Saab specialist and he said the DI cassette is throwing error codes because some of the electrical components inside it are going bad. Is this a major concern? Should i change it immediately or can it wait? Also, is it a worthwhile investment to go a salvage yard and see if i can find a used one? Other than that the car runs great, no complaints. Any advice is appreciated. 

Also, spark-plugs have not been changed (due at 90K miles, right now i am at 87K). When i bought the car it was at 83K earlier this year, don't know what kind of service was done before i got it. 