general problems of the 2000 Nissan Quest

Once my car's warranty was expired I began having problems with the "engine" so I thought. The check engine light came on and the car began running rough. I took it in and was told there was a knock sensor malfunction and a misfire in the #6 cylinder. I took it into the Nissan dealership because the car completely died. I authorized the spark plugs to be replaced and the battery cable if it was necessary but was told the spark plugs are good for 100,000 miles and the battery cable was fine even though the original one split in two killing my car. I was told the knock sensor needed to be reset and my battery was dead. I am still having the rough idle so I took it in again and was told the knock sensor again had a malfunction code and the computer is telling the O2 sensors to run leaner and the O2 sensors are telling my computer to run lean. Well now there is a nasty sulfur odor which in the past has meant the catalytic converter. No one knows what the heck is wrong but it still (a year later) runs rough with the occasional feeling it wants to die. I need help, I believe the catalytic and the O2 sensors are going bad.

After engine is running for over 500 miles,(ie road trip)the van starts to buck and buck, engines hesitakes,no power is this a engine vapor lock problem. Dealer could find no problem with engine test machine.