ya that deffenatly is the problem, i had similer issues, i just finsihed my header install and wanted to hear them just straight headers and the o2's werent in and it ran good cold and i took it down the road cuz i was chasin my friend and it started doing exactly the same trhing as you describe, put the o2's in and whammo!it dissapeared!! good luck , nice sounding car by the way

 

Thanks, I hope this clears it up.