I haven't tried anything yet. It feels like some of the cylinders misfire.

I will certainly try fuel additive, but what do you think could be the root cause? I have the sense that it's part of the fuel delivery subsystem, but I don't know the particular components well enough to venture a guess ...

I'm getting a consistent sputtering or lugging when I push the throttle and the engine is between 500 & 1500 RPM. It feels like the engine is misfiring: jerky and rough.

I have a manual transmission, so I know it's not that. This happens after I put it in gear. For instance, if I'm in a high gear ...