Jeff couldn't it be fuel pressure related in theory? Kind of like it will start to misfire on one cylinder and not be a random miss if the pressure is only slightly too low. If he could look at the trims, it's possible the other bank is running lean, but not enough to set codes.
Anyways Jeff is right. It's probably a vac leak that only effects the bank that's running lean.
2008 Mercury Sable Premier
2014 Ford Focus Electric
1997 Mercury Grand Marquis GS
Moved On: 2000 Sable LS Premium Duratec