Third cylinder misfire in my Nubria

My girlfriend has a 1999 Nubria with a misfiring problem in the third cylinder. The engine was rebuilt and has low miles. I was wondering if anyone could give me some advice as to how I might approach this problem without having to throw it into the hands of an expensive mechanic right off the bat.

Have you solved the problem with the misfire? I have a similar third cylinder misfire when cold in the same model vehicle- but as yet no solution