02 Ford Taurus Cylinder 1 misfire code with DPFE low voltage code

I have an 02 Ford Taurus with the "U" engine that
has been acting up ever since i started driving it
again. I retreived the codes when i first started to
have problems with it and they turned out to be a
misfire on cylinder 1 and 4. I checked the spark on
the coil packs and ccylinder 4 turned out to not be
sparking at all so i replaced the coil pack and the
wires. Cylinder 4 misfire fixed. Cylinder 1
continued to misfire so i went ahead and replaced
the injector. The car seemed to be running fine
after that. Cleared the codes but the DPFE low
voltage code still popped up and i can hear what
sounds like a soft clicking noise coming from the
intake manifold area that can only be heard when
the hood is open. Other than that code the engines
runs fine... until i took it out on the highway for an
hour drive recently. She ran fine on the way to my
friends house but on tthe way back the CEL began
to flash and i lost power. So i got off the nearest
exit and popped the hood see if anything was
loose like a plug wire or something. Everything
seemed ffine. Got back in revved the engine a little
bit in park and it was running just fine again.( I
noticed when i firsted got off the highway and
revved the engine a bit the car seemed to chug at
about 2000 to 2500 rrpms) so i got bback on the
highway again and it was fine for a couple miles
then the same thing happened again. Got off the
highway did the same thing put it in park opened
the hood, got back in CEL ceased to flash and the
car was running fine. So i took the back roads
home because obviously the car wasnt liking the
highway. It ran fine all the way home. I pulled the
codes again and what do you know it stored a
cylinder 1 misfire code and the DPFE low voltage
code. I am confused at this point. I have a feeling
the DPFE sensor is ccausing the EGR valve to
malfunction but i want to make absolutely sure
thats the problem before i go out and buy a new
one. The clicking noise coming from the intake
manifold also has me worried. Im afraid its a valve
tap or maybe its something also being caused by
the malfunctioning EGR valve. Any input would
be.greatly appreciated. Im really in need of some
opinions because nobody around me knows what
im talking about. Very frustrating. Thank you so
much.