Got the same problem sort of, after a long hwy drive and I come to a stop the gauge flutters, I guess this thing is no sender but just a switch and the guage is a fake lol. you might check around the oil filter, many times there is a plug there someplace.
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.