I'm battling the same code in my daughters Zetec. Since you replace the O2s & MAF already there are two suggestions I can offer you:
1. Air leak, check as stated above. Also check the vac tube from your brake booster, as those have been known to crack or develope holes.
2. You have a '96, which have had wiring problems. check the wiring to the appropriate bank & O2 and see if the insulation is in good shape, unplug the O2 and clean the contacts within the connectors as best you can.
Best of luck
