I need help...1998 F150. Showing P0303 Cylinder 3 Misfire code. Replaced Spark Plugs(me), Wires(Mechanic), Coil Pack(me), EGR(me), Fuel Injector(Mechanic). Compression test and other tests good(Mechanic). Reputable Mechanic did the fuel injector, drove it and said it ran great for 10 minutes. He turned it off for an hour. Fired it back up and drove it for 5 minutes and it started missing again and throwing the same code. He gave it back to me and says he can't figure it out...HELP!

 

Which motor?

 

Assuming you used all Motorcraft parts....

With all the components replaced. It may need to go through 3 drive cycles before the computer adjusts itself. When I had a misfire, replaced plugs and wires, it took a couple of days before it started running smooth again.

 

Manually test the injector in #3
"Did the fuel injector" means what exactly
If replaced, was it a Motorcraft?

 

The mechanic replaced the fuel injector. I used parts from OReilly. Not sure the brand.

 

4.2

 

Look at the wire harness female connection VERY close. I find that after that many years they loose their elasticity and will not make contact.
You could take a large needle and try to close the copper contact so it will touch or buy BRAND NEW connectors on line and splice.
Or push the male pin over a bit and see what happens.