Car starts "slipping" after driving for a while and one of the fans doesn't work ..

CEL light read the P0720 code as well as P0420(bank 1) & P0430(bank 2)

which sensor do I need to replace?

autozone CEL/SES receipt says "vehicle speed sensor" but I have also found the "existence" of transmission revolution sensor

any website suggestions for purchase other than local stores and dealer?

 

I am having the same problem with my 02 maxima. Codes 0720 and 1574 (among others, but they are unrelated, we think). I have not done anything to fix it yet, but am strongly leaning towards replacing the speed sensor (or maybe both of them??) with a nissan part from ebay (cost: $47 versus $120ish from autozone or equivalent auto parts store). I found this thread helpful...maybe it will help you too. There are some links within this thread showing pictures of the part, too, which is helpful.
http://forums.maxima.org/5th-generat...lp-please.html