Today it happened. I was flying a PA28-140 that belongs to a friend. I pulled it out, preflight was fine, runup was fine, everything in the green. The takeoff was fine, making full power and the climb out was typical. At 500' agl I begin my crosswind turn and the engine stumbles for a second but only a second and resumes full power. I continue my turn and at about 600' I lost 500 RPM and the engine began to run rough. It was still making power, just not full power, I continued my turn back to the runway. I announced my intentions and landed without any problems. I turned off the runway and began to attempt to find out what was wrong. Oil pressure and temp were fine. Fuel pump was on. I had pulled carb heat during the event, there was no change. On the ground it idled fine and a ground run up only got a very slight almost not noticeable miss. However, when I would go to full power, I'd only get 2000 RPM and a very noticeable engine roughness. I leaned aggressively to clean the plugs, that's not it. I parked it and looked everything over and can't find anything that jumps out at me. I probably left some details out, but what in the world could it be? I flew it last weekend and it was fine. The other guy flew it Thursday and it was fine.