I have a 07 G35 6MT and sometimes - maybe 1 out of 50 starts, the car takes very very long to start. I can hear the engine turn over 20-30 times fast (this must mean the battery is good ) but it won't start. The first attempt I give up but the second time after a while it starts but it's not a 'clean' start, it sounds like it starts at very low RPM and I feel the need to give it some gas.

There's nothing particular about when it happens. The car hasn't been sitting idle for too long, it's not colder/warmer than other days, etc.

What could it be ?

 

fuel pump or fuel pressure regulator, maybe a messed up TPS?

 

Cam and/or crank sensor is a possibilty. Its very common on The 05ish altimas. 9/10 times thats what it is when altimas come into the shop with a long start condition. Service engine soon light on? I had to replace the cam sensor on my 04 coupe.

 

Possibly your battery.