Yup ford has wired our op gauge so that it's nothing more than a mechanical idiot light!!!!
When the op gauge reads low, does the op warning light turn on???
If you've replaced the op sender, then check it's electrical connector for loose fit, or corrosion & it's wiring for broken strands.
If thats ok, did you use PTFE ribbon dope on the threads for a sealant???? If so, it may not be making a good ground connection through the threads to the engine block. so maybe check it with an ohm meter, or connect a ground wire from the chassis to the sensors metal casing & see if it stops acting out.
Let us know what you find.