OMSI stutters, it's not a well made game.
However, if the stuttering you are referring to occurs as a single big pause regularly while driving, that's because OMSI is loading more map. You can reduce that by selecting the "Load whole map at start" option in the settings (which will likely reduce your overall performance), or by running OMSI off of a faster hard drive (i.e. a fast SSD).
If the stuttering you are referring to is simply the fact you have 5-15 FPS, then you can fix this easily by either turning down settings or buying a better PC.