Ramdom exposure with flash

Hi i have a d600. I do not shoot in manual mode as much as i like to.
I have recently went to a winter fest, took my camera out after a long time.

I was taking pictures of my friend's kid with the 50mm 1.8 G lens i had in auto mode ( i did not try to use manual mode).

I problem i had is some of the pictures were randomly over exposed and some of them are under exposed. Rarely exposed correctly.
Doesn't auto mode mean the camera has to take care of all setting for correct exposure?
All of the pictures are taken with on board flash. Is something was wrong with the my camera?.

I went home and did some testing i have took some pictures in manual mode with out flash and so exposure fluctuations. Same in the auto mode with out flash.

But as soon as i switched on flash the pictures started getting randomly over exposed again in both manual and auto mode.

Am i missing something did i mess thing a setting i am not supposed to? .... i was doing a lot of learning on this camera few month's ago.
I messed around with a lot of setting's.

Thanks,
PoolKolli

We need more info. What metering mode were you in? Did you have the flash set for TTL? So many things can cause the problems your having. Try posting some images.