I installed new rotors and brake pads and broke them in properly for about 200-300 miles and i don't know what's the problem.

If i'm going about 70 and i start to brake slightly and i hold it in more place for a nice slow stop the front of the car begins to shake and it sounds like it's grinding against the rotors and i checked everything and nothing seems to be wrong, I took off the caliper and checked the pads and piston and everything and i can't find nothing wrong.

The car brakes very good dispite the shaking but i have no idea what's wrong.

Can anyone please help me to get some questions on what it could be and it only dos it sometimes and other times it's fine.

Did you torque your wheel lugs correctly?

It IS possible that your rotors need to be trued to your hub. Some dealers will grind new rotors lightly after they are installed to make sure they are 100% true to the hub. I know Honda does this to new rotor installations, Nissan probably does too.

You may need to have Nissan do this service for you. They don't take much meat off them, it's only a very light grinding to true them up.

Should cost around 60 bucks.

Sounds like warped rotor? (If you feel the vibration in the steering wheel). Aside from checking the lugnuts as already suggested, you can check for warped rotors. Lift the front up and spin the tire. If you hear an uneven 'scratching' sound at a certain position each time it goes round...

The grinding sound when you are braking is fairly normal until the pads wear flat onto the rotors after several hundred miles.

you night need to bleed your brakes but it sounds to me like warped rotors.

Thanks alot for the feedback guys and i just had an alinment done so it can't be that and the steering wheel isn't vibrating that much but it's a funny noise and it's acting like it's griping and releasing as i come to a stop.
I have crossdrilled rotors and EBC green stuff pads if that helps and can you have crossdrilled rotors grinded to the hub?

Also i just tightend the lugs using the old foot on wrench method untill it can't go anymore.

definitely seems like the rotors are off. Perhaps the pads aren't seated properly...if you just had the service done not too long ago, you should take it somewhere else and have them look at the installation!!! Sounds like a bad install.

Sometimes rotors just warp, but if it was recent and you broke them in, then....

Thanks alot everyone.