Onan Generator Hesitating or Stuttering

I just acquired this genset. Its an Onan Cummins HDKAT 7.5kw Diesel Generator. Its got a lot of corrosion due to rain but it runs ok for the most part. I had to replace the battery charger and replace the ends of the wires on the transformer due to corrosion just eating through the connectors. Its charging the battery great now. My problem isn't a huge one but an annoying one, that I don't want to cause other problems or get worse. I was told the genset was fully serviced before I got it, but it did sit for a while (unknown length of time) un-used and I doubt the fuel was treated in any way. I'm not sure if it was actually serviced or not but as I said the genset starts and runs well other than an occasional stutter or hesitation about once every minute or two. With no load its barely noticeable while standing next to the genset except the puff of black smoke emitted from the exhaust. Under 90% load it stutters a lot less often but when it does it's hard enough to shake the trailer and the lights dim. It is not a load kicking on or off. The only variable loads are the air conditioners and we don't run them often. The fridge is non mechanical, it runs off of a large peltier unit, its load is barely noticed by the genset. Could this be due to a dirty fuel or air filter? I read somewhere I may need to adjust for altitude but that may have been on a gasoline set. I'm not sure if that's something I can adjust on this set. I don't want to do too much without some guidance. Thanks in advance for any help provided.