94 Impreza Rough Idle and acceleration

I have a 94 Impreza with 105,000 mi. It's the last of the FWD Subaru and has a 1.8L, 4 cylinder engine.

The problem is that my father changed the sparkplugs. It worked fine before he did, and he did this to improve mileage, etc. The car worked fine driving 150 miles to where I live from home after this repair. It worked fine on the two 20 mile drives I did this past week.
I tried taking it out the other day and it started up with a rough idle. Then I didn't notice anything until I got off the dirt road and onto the pavement that it started driving strange. We noticed the front end felt rough and choppy, similar to when tires get out of balance, but it was more engine related. It felt like it did when idling. Perhaps he didn't wire the plugs exactly right, but I don't understand why it took a few drives for this to happen if so. The car then smelled like burning rubber for a few minutes. We stopped to check it out and it idled rough again. The shaking feel of the front end/engine was more noticable in idle and when accelerating. I ran it today in the driveway and it still idles roughly.

I'm thinking maybe something is up with the headgasket, so I'm going to take a look for some other symptoms. I was wondering what others thought before I take her to the local mechanic.

thanks!