Recently, I was climbing out in a Cherokee 160, P-28 I have been renting when, at about 300' AGL there was a sudden, marked decrease in power. It sounded as if someone had pulled the throttle back about 25% and the engine started running roughly. I thought it was going to die compelely. I immediately checked the carb heat, mixture, and fuel pump while turning back to the runway while I still had some power. After about 15-30 seconds the engine regained full power, but still did not sound right. I landed safely, returned the airplane to the hanger and contacted the owner. Let me say, I did a complete pre-flight and run up, which included checking the fuel for water at all 3 drains. Magnetos and carb heat were functioning properly before take-off. A mechanic checked the plane and said the lower plugs were fouled. I was told he corrected the problem and tested the engine and that the plane is now "good to go." Honestly, I am not excited about getting back in that plane. Does the mechanics explanation sound reasonable? What precautions should I take or what questions should I ask before trusting my life to that same plane?