Trouble shoot Cold miss for 2 blocks

[bcredliner]

I have very slight miss or surging from a cold start but only discernable
for 2 blocks. If it is still there after reaching normal operating temperature I can't feel it. There is no fluctuation on the tach (not very sensitive),I don't feel a lose of power but sometimes there is a slight delay in trottle response. I always have the AT in manual so it is in 3rd. gear at about 2000rpms. No codes show on my Bavauto Peake equivalent scanner. I am leaning toward an O2 sensor because the milage seems to have dropped offbut as you know it could be one of several other or a combination of possible causes. I don't want keep driving until it gets worse as I am concerned about ruining a cat. 108,000 on the engine. Question is---how do I determine the cause without throwing parts at it. I am kindaOK paying the dealer to diagnose the problem.. Can I get the information with an onboard unlock?