How do I restore power to my 150 cc scooter?

I recently bought a Prodigy 150cc scooter. I bought it from a guy who left it sitting in his garage for about 2 years. He said that he hadn't really drove it. It only had 17 miles on it. I had taken it to a local scooter shop to have it serviced and they cleaned the carb and put in a new spark plug. I put about 150 miles on it and it ran fine. One night after I had been driving for about 5 minutes i noticed that i was having trouble getting up hills. then I couldn't get over 25 to 30 miles an hour. What could be the problem here. The scooter has about 250 miles on it right now. It starts and idles fine, but when I try to drive just no power. The engine seems to be a little sluggish. It runs steady until I try to open up the throttle and then it starts to miss and cut out. Anyone have any solutions on what to do????