Volvo MD 2030
 

It sounds like you have an injector that is hanging up, causing that
cylinder to not fire. You can isloate it if it continuse to happen.
Just crack open the injector lines one at a time. You should see a
change in RPM's each time you loosen the fuel line on each of the
injectors. If you find that one does not change the RPM's when
loosened, that's the bad one. If you can wait until the end of the
season, I would bring all the injectors in for servicing at the same
time. The tips are probably clogged.