1998 Lincoln Navigator engine stumbling

My navigator started stumbling on acceleration about 3 months ago and has gotten steadily worse. The stumbling initially only occured on acceleration and only between about 1100 to 1800 rpms. It was most noticeable on inclines at about 45- 50 mph at about 1200 rpms, It has recently started stumbling continually from 900 to 2300 rpms. It idles fine and at higher rpm ranges it runs perfectly. The check engine light has never been on and there are no codes. I have replaced the air filter, fuel filter, and mass airflow sensor with no success. The throttle position sensor tests well with gradual resistance changes on opening the throttle. The plugs were replaced with platinum autolights about 25,000 miles back.