outstanding advice from Michael, as we only expect.
as for the ECT, you asked,
"i mean, how can you get the voltage readings back within specified range?"
as i understand, its more a matter of how well the sensor is conducting at specified temps. they get old (i dont know on a degredation level why), and dont read correctly. i would say to get a new one at the parts store while you are there. should be under 10 bucks. while i agree with Michael on not blindly tossing money at sensors, im not sure he would disagree that since you are getting the code, and you admit the sensor is of an unknown age, to go ahead and stick a new one in. so cheap, why not (my thinking).
as for the plug wire thing...did you do an additional test? i use the test i gave when ruling things out. once a bad wire is found, further investigation is usually performed, as Michael mentioned (ohming it out, etc). you could swap two wires (swap on cap and plug), and see if the same wire is still intermittant. i know your wires are free, but doing further tests would confirm your culprit, so if that is not the problem, you havent wasted any time (i hate having sick cars). i would get some new wires regardless. just a good idea, and the price is right.
good luck