CaymanS2k

when driving and reving higher rpms there is a plopping sound coming from the engine. any idea what this could be? also car feels a little sluggish when taking off. engine was just completely rebuilt. only thing i can think of is the spark plugs for now. i have the oem plugs in. but they are not exactly new. have about 50 miles on them. took them out of my old engine. i assume the vavles are adjusted fine from my machanic. so what else?

please give me some ideas...thanks