Alternator going????

[crazchris72087]

My 97 Maxima about 1 1/2 to 2 years ago made a weird starting noise. Everytime i turned the key it would crank though. But sometimes when i cranked it it would start to crank normal than out of nowhere go offbeat to the reg crank noise and then finally starts and a lil gray smoke comes out of the exhaust sometimes (not real bad at all though). I got my starter replaced around that time and my car cranked up fine for the next 2 months and then ever since then till this days its cranking sometimes again with hesitation. What do yall think could be the problem to this? My battery in my car is only a year old and tested it and its fine. Any suggestions? Would my alternator be making my vehicle crank like that and if so how come the alternator hasnt given out since its cranked like that for almost 2 1/2 years? Thanks guys.