I realize this is an old thread, but it popped up when I was researching problems with my 2000 Sentra GXE. I solved my problem with the help of this forum and others, and many youtube videos, and wanted to pay it forward for anyone else who is having a similar issue. My sentra's idle was dropping on 15 second intervals causing it to shake quite a bit, as well as sometimes stalling at stops. It also hesitated on acceleration, had some shifting issues, and it did seem like running the ac helped some. The only codes the computer had were for the O2 sensors, and, on inspection, the dealership tech said the cat was breaking apart. Long story short on that, and 1300 dollars, all new O2 sensors and a cat later, none of the problems cleared up.
I decided to take matters into my own hands at that point. I did a lot of testing and diagnosing, but want to keep it short, so I did just about all the standard tune up stuff and replaced one bad ignition coil. It all helped with performance, but didn't clear up the issues. While continuing down the diagnostic check list I drained down the coolant enough to pull the thermostat out. The thermostat was stuck ever so slightly open. I replaced it and topped up the radiator and reservoir. The idle problem actually got worse in that I had gotten it to stop stalling, and now it was stalling again. I checked the reservoir, and the level dropped, so I filled it to the max line again. I did that one more time, my assumption being that there was still air in the coolant system, and it was purging through the reservoir. After another day of driving, the idle problem cleared up, the acceleration issues disappeared and the shifting problems went away.
Thinking back with this new perspective, I know the dealership (whom my wife insisted on using for the regular maintenance) had serviced the coolant system, and when I started doing my diagnostics and tune up, I know the reservoir was dry, so I topped it off. My assumption is that the dealership failed to purge the air when they did the service. I attribute the fact that it didn't self purge after I topped it off originally to the bad thermostat, and the fact that it was stuck open prevented enough pressure build up to blow the air out through the reservoir.
A lot of this explanation is speculation, but it's the only thing that makes sense to me. So, if your 2000 nissan sentra (and maybe up through 2006? And maybe maxima or altima) is having similar symptoms, do all the tune up stuff, then check your thermostat before moving on to more technical diagnostics. This was literally a 20 dollar repair.
On a side note, I had pulled and tested the egr valve because the first google hit that popped up when starting my search suggested the egr valve could cause all the problems I had. I didn't have much to go on for testing, as all of the videos I watched were for the older style, vacuum driven egr valves. The sentra's egr valve is solenoid driven/computer controlled. I tried testing the solenoid with an 18 volt battery and some alligator clips. Regardless of the configuration of leads on pins, I could not get the solenoid to activate. I assumed it was bad and replaced it, but I could have been testing it incorrectly, and replacing it did nothing to help with the issues. The only other confirmed issue I have is the MAF sensor. The computer did eventually throw a p0100 code. I tested the voltage through the MAF sensor, and it starts at 3V and drops when opening the throttle, whereas it should start at 1V and increase. The factory service manual confirms a bad MAF sensor for a p0100 code with high voltage output from the sensor. The car is not having any noticable issues now, but I will be replacing the sensor, and hope to maybe improve the fuel economy some. Finally, I periodically pulled incorrect gear ratio codes, which got me worried that the transmission was going, but since replacing the thermostat, all shifting is good. Hope this info helps someone out there.