If you removed the Temp Sender on the intake, it has ONE wire, did you happen to put sealant tape on it's threads when reinstalling it?
This ONE wire sender uses the intake(engine) as the ground in the circuit, so it needs at least the lower threads to be bare for a good ground.
(There is a TWO wire Sensor that looks similar to the one wire sender, it is for the computer, so not used by the temp gauge)
To test if temp gauge and wire are good, turn key on and remove that ONE wire from the temp sender, gauge should go all the way down, now get a short wire and Ground that ONE wire to battery or engine, gauge should go all the way up.
If this happens gauge and wire are OK.
Also there is a Ground strap from drivers side head to the firewall, make sure that got hooked back up and is tight on both ends, this is a main ground for the instrument panel.
Your year used a MAP(manifold pressure) sensor to set baseline air/fuel mix, it is on the firewall passenger side, it has a vacuum line and electrical connector, it is often hidden behind wiring harness.
Remove and check the vacuum line, remove and inspect the electrical connection for corrosion, where it sits can get water intrusion.
MAP sensors rarely fail.
Could be fuel pressure issue, should be between 35-45psi, if you do rent a tester run engine at 2,500rpms for a minute or two and see if pressure starts to drop, on older vehicles the screen(sock) in the fuel tank can get clogged up which lowers the volume of fuel available when demand is high, pressure looks fine at lower RPMs but will steadily drop at sustained higher RPMs.
Make sure to use Motorcraft spark plugs and gap them at .044
Get spark timing light and test timing with spout connector off and on, then rev engine and see if timing is advancing as it should.
Base spark timing(spout connector removed) should be 10deg Before TDC, some thing 12 deg is better.
Good read here on TFI system:
http://www.therangerstation.com/tech_library/TFI_Diagnostic.shtml
If your spark isn't advancing as it should power curve drops off at higher RPMs.
If you used headers to get better low-end power that power is taken from the top-end, headers don't "add" power, they move the power band lower or higher, but that doesn't read like your issue.
One thing you might check, is if your year and engine has a Knock Sensor, I am not sure???
A knock sensor will cause retarded spark timing if "it thinks" the engine is pinging/knocking, this will rob power at higher RPMs.
I have read some '86/'87 2.9l engines had these, usually on starter motor side just above oil pan.
read here:
http://www.therangerstation.com/forums/showthread.php?t=91259