Q: New EGR Transducer Leads To Other Issues?

Hi! I drive a 1998 Subaru Legacy Ltd 2.5 ltr and have for about 5 years now. Recently, because of its age, I've done a lot of work on the engine--most recently, a new EGR transducer. Since the transducer was installed about a week ago, I've had nothing but problems. The car shakes at idle, so I figured the EGR valve itself or the EGR solenoid needed to be replaced next. My mechanic assured me it was the EGR valve, so that was replaced yesterday and the codes cleared. But when we cleared the codes, a new one was present-- cylinder 3 misfire. The only code present when the EGR transducer was replaced was the EGR code. The car still shakes at idle intermittently with both the EGR and cylinder 3 misfire. The next step I think is replacing the EGR solenoid is the next step, but what could be causing this misfire? Spark plugs were done about two years ago, so it might just be time for that--and easy fix. Thanks!

My car has 180000 miles.
My car has an automatic transmission.