Toyota Repair: Toyota 4-Runner 22RE Check Engine Light, oxygen sensor, pace setter

Question
Hello,
The problem is on my 1987 Toyota 4-Runner 4x4 5-speed 22RE fuel injected, 150,000 original miles, 5,000 on new overhaul.
The problem started after I installed a header made by Pace Setter.  The original location for the O2 sensor is on the exhaust manifold close to the head.  The new location for the O2 sensor is behind the header collector under the cab.  I extended the sensor wire with similar sized wire.   Now the check engine light comes on intermittently.  It only comes on when I am traveling at a constant speed when the revs are above 2500 rpm.  If I let off the throttle, the light turns off after a second or so.  When I get back on the throttle the light will come back on after 10 to 20 minuets.   The light has never come on while idling.  Also, sometimes it will idle at about 1500 RPM, sometimes the idle will be so low it stalls, other times it will fluctuate, sometimes it will idle fine.  I installed a new O2 sensor and it didn't change anything.   I read the stored code and it gave me 5 flashes.  I looked it up in Chilton and the explanation is "Oxygen sensor signal-sufficient feedback but oxygen sensor signal unchanged".  I have noticed an large increase in fuel consumption.  If you need any more info, please let me know.   Thank You.

Answer
well, I think the main reason is the new location of the oxygen sensor and increase of the sensor wire, although the size is the same. Increase in length of wire can increase in resistance and effecting signal. And the ECU is not getting correct information from the oxygern sensor. And as a result the fuel consumption is increased.