After warm horribly rough with misfire

Engine runs and idles fine until coolant temp reaches normal range then like a switch all goes rough. Idle is rough, acceleration is as if on 4 out of 6 cyls. Back off throttle and it clears up. Occasionally problem completely disappears for a day or so.

Codes said misfire of #6 (I've replaced plugs), bad O2 sensors (I've replaced both). Problem persists. ECT sensor or something??

Could be a dirty MAF or an EGR (sticking on) issue.
G/luck
Joel