Hey Rocket! And Joe! I had changed the o2 as part of the all the componet swaps, here is anew one now in place. I've not yet installed an AFR gauge. I'm suspecting the stumble or stutter is leaness. I've changed dists and checked for advancement through RPMs, OK. I,ve changed ECUs also. Have not installed your rocket-chip. Am waiting to get this problem solved, then will install chip. Changed fuel pumps but there is no problem above 3000. It's because of it running good when cold is why my question of this sensor. Guess I'll buy a new one and get to work. Thanks Guys. Gus