Scag Liberty Z surge problem

My liberty Z is in it's 2nd season. I have it serviced at the end of the season so i can get a jump on spring. This year the first mow went without a hitch, about five days later while mowing, the engine began surging. Low rpm then normal rpm. The gas was new and I even added a fuel stabilizer to the tank. The problem seems to only appear when I have the blades engaged. I have visually checked under the deck, and checked all belts pulleys etc. I clean the mower after every use to be sure no grass is anywhere on or in it including under the seat. Any ideas??

What engine is on your unit. Need numbers to be able to help much farther. could be a fuel delivery issue or plugs, all the way to coils and the push rods. But with your comments I would have to say fuel delivery at this point.