Battery advice please

[delorean_dude]

Hi, I have a Lazer zx-5 car which i love. it has a hobbyking brushless motor and speed ctrler etc on. the batteries i got with it are 25c 7.4v(2 cell?) lipo. When i take car out it goes fast for a bit. then about half speed for a few secs. then stops. pauses then shoots off at full pelt. im not sure if its the vattery or charger? or car? dont think its the car. any ideas please?

also if need new battery and charger whats diff between 2 and 3 cell and any suggestions on a good battery and charger to get? i dont have as deep pockets as id like.

thanks