Hi Bill,
I have had long termp recption problems in two instances during the past few years on AM. Both times it has been traced to switching power supplies.
One power supply had a strong peak at 690KHz, splattering nicely to 740, and much of the rest of AM, too. You may have a switcher with a peak at 740.
Think of what has come into the house lately, and been plugged in.
Best Regards,
Bill