Shaking while Idling

I have a 1995 Subaru Legacy Sedan and, granted, it is old, but has never run poorly EVER. Recently, it shakes/vibrates a lot, especially when idling and it doesn't idle at a constant speed. Sometimes it sounds like it's going to shut off. It isn't noticeable when I'm driving or when I have the car running in park, but when I'm stopped in traffic it gets worse every time. My check engine light has also started coming on every other day or so, but I'm not sure if that's related or if it's a faulty sensor, which is what the last mechanic told me. I don't have a whole lot of money available to me at the moment and have a four-hour trip to Maryland planned for the end of October, so I guess I'm looking for some answers that don't cost an arm and a leg. Thanks :)