starting about a month ago, when i put my car in gear, sometimes it starts to stutter from 1st gear all the way to 6th. it does it from about idle to 2,000 rpm and it just feels like the engine is struggling to get going and i have to floor it to get it to stop. And then it idles quite irratically. it'll go all the way down to 500 RPM and then rev itself up to 1,300. The car is almost undriveable sometimes but it only does it sometimes so when i took it into my mechanics shop, it didnt do it at all for him so if anyone has any ideas on what the hell is going on that would be great because it it really starting to annoy me quite badly......

The car is a 98 Trans Am WS6, M6, True Duals, and Pacesetter Headers