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P0300 & P0303 codes come back regularly, what else can I check?

(2007 Ford Taurus)
in Perry, IA on May 10, 2012
2007 Taurus 3.0 V6 - 125k Hwy miles and has become hard to start warm and cold. The P0300 & P0303 are regular, P0174 popped up and went away after about a week.(check codes and clear daily). The fuel pump does growl with the key on after it sits over night but it does charge the system at 39psi when the P0300 & P0303 is set, RPM's range from 700-1500 when code is set.

I have replaced Plugs, wires (50k miles on them) & coil (orig w/hairline crack), Cyl 3 plug was discolored but not fouled or built up with residue. Other plugs looked clean. EGR tested OK on the bench and on car w/no carbon build up (did not test MAP sensor), cleaned MAF, sprayed intake with Carb cleaner w/no increase in idle. Tested Cyl 3 Injector with VOM and is with in spec of the other injectors. The mis comes and goes at idle. The IAC motor appears to not zero out when ign is turned on it does move when visually inspecting it. When idling, the IAC voltage varies from 0.8 to 0.1v. Cyl 3 has 145 psi of compression.

What do I check for next - what direction can you give?
Thanks Brian P
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