From the discription of your plugs, it sounds like you car is running rich. My first question is What kind of plugs are in there? You should be running NGK Platinum plugs. They are OEM plugs and are hands down the best plug for the car. USE THEM! Bosch makes a replacement plug as well but our ignition system EATS the electrodes on Bosch plugs. Don't use them.
Your "tap" on the MAF sensor which caused the car to cough is probably a good sign it's on it's way out. You can clean it, use some alcohol on a q-tip or a squirt of Air Intake Cleaner will do the trick as well. MAF sensors are expensive, like 400 bucks. Search the web for discount Nissan parts. Courtesy Nissan is a good place to look, and if I remember correctly, Discount Nissan Parts - search that exact phrase has cheap MAF sensor prices. You may want to do a search on Maxima.org for MAF sensor. Some guys on there have replaced it and know where to find them cheap.
A few other things you may want to take into consideration -
When was the last full tune up? Every 60K you get a major service.
You back flushed the fuel filter? I'd replace it if I were you after that trick. If any particals get into a fuel injector, open your wallett and dig up $$$ for a new injector.
Clean your throttle body.
www.motorvate.co has a nice write up with pics on how to do this.
Tp pull codes from your ECU:
To the right of your gas peddal sits the ECU. You have to remove the plastic trim to get to it. There is a hole in the side of it with a screw in it. Use a flash light and look into the hole to see the screw head. That screw is how you get codes and reset the ECU.
While the ignition key is in the on position, car NOT running turn that screw head clockwise, then counter clockwise. Look at your CEL light on the dash - if there are any codes it will flash. Watch the serier of pulses, there are long and short pulses seperated by a pause. Then it repeats again.
2 long pulses followed by 3 short pulses would be 0203 and so on...
Write down the code that your ECU spits out, if any and I will tell you what it means. Code 0505 means everything is OK.
To reset your ECU, you turn that screw back and forth twice within a few seconds. That clears any codes in your ECU, it also turns off the CEL light if it's been on.
For more information, check the following web sites -
Maxima.org
motorvate.ca
maximadriver.com