Nissan Repair: Inconssisent start ups, oxygen sensors, nissan maxima

Question
I have a 99 Nissan Maxima. I had a problem with it awhile back with starting, it would turn over but if it did start it would shut off shortly after. It wasn't an idling problem, gassing it did no good. I attached a machine and got a p0420 which was a catalyst system fault (right bank if i remember correctly) and I ended up replacing one of the O2 sensors which seemed to fix the problem...temporarily. A week or two later i was parked and tried to start it and same thing. shut right off. it has always run with no problems. I took it to a shop. they though it was the MAF sensor, and even though i didn't believe it (Tested it my self...) I let them replace it. Car started up again. ran rough for a day and then seemed like things were gonna be good. A couple of weeks later. SAME THING. i was getting frustrated. It was left alone for several weeks at a friends house ( i was visiting him when it wouldnt start) and as a joke i tried it. It has been running since, if at times a little spottily. Its been over a month since it last started and its been the longest its run (Since i bought it to when it first broke, even) I love the car when its working. Its smooth and powerful and is an all around nice drive. Im just concerned its going to happen again and would like some confidence. im no longer getting an error from the computer (No CEL). I had a problem previous where the alternator went, stopped working for a week or so and hasn't given me issues since. I'm considering the ECU, but I really dont know. Thanks for your time

Answer
Joshua,

I would replace all the oxygen sensors.  There are three.  I have a 2000 Maxima SE and I did what you did and only replaced what was throwing a code and ended up replacing them all and the car ran perfect.  Also, if you have not changed the plugs in a couple of years I would do that too.