One of the things that my mom noticed within a few days of driving was that her durango would suddenly begin to use a tremendous amount of gas without warning, causing her to think that her install was done improperly or that the construction of her hydrogen generator was done wrong. While she was right to go back and check her system, she forgot to add one simple part to her generator system - a Dual Edge MAP/MAF Sensor Enhancer.
When you installed your oxygen extenders, they helped your car to run lean and "trick" your O2 sensors into not pumping so much gas into the system when it senses more oxygen coming in. Even with the oxygen extenders installed, your car will occasionally sense more oxygen coming into the system and tell itself to pump more gas in to compensate for that.
The Dual Edge MAP/MAF Sensor Enhancer is a simple device that you can install yourself if you're comfortable with working with vehicle electronics, or you can take it to a mechanic to set up. It helps your car to omit that false signal that causes your vehicle to pump in extra gas when it isn't needed.
This website explains what is going on to give you a better understanding of what the Dual Edge MAP/MAF Sensor Enhancer does and why you need it:
http://www.h202hho.com/products.html (Scroll down and pass all of the pictures)
Here is a video that describes the installation in more detail, posted by Skyteck5000 on YouTube: