Since you mentioned a diesel mechanic; I'll assume you have a Liberty Limited or Liberty Sport with the 2.8 liter turbo diesel motor. From the mileage figure you mentioned, I'm also assuming the year of your vehicle is about 2005; but it sure would have been easier to give you an answer if you had at least posted the model year and engine type/size of your vehicle.
The P0300 code means that more than one cylinder was misfiring at the time the code was set. This could certainly have caused a loss of power. The most likely causes of this code, and the loss of power, are either a defective or dirty mass airflow sensor; or a clogged fuel filter.
One simple way to address the possibility of a mass airflow sensor problem would be to have the MAF cleaned. I would also suggest replacing the fuel filter.