Erratic idle....

Hello forum, I haven't been in here for awhile. But I have problem, it's 22:38 and my 00 firebird 3.8 is acting up. Its like one coil pack is out but all three are working fine (checked and they have spark). Switched the map sensor and its not it. Reading on the interweb lol nah jk, the internet, and I came across the crankshaft postion sensor (its throwing a P0336 by the way), not sure what to believe. This is a "new" motor on the car, the orig. took a dive at 204,xxx mi. this one I got with 53,xxx mi. on it, It just seems hard to believe that that sensor went out this early, since on the original it was working fine. Shine some light on this please and help me cuz I really dont wanna tackle that cranksaft pulley.