i have a 10' gt3 with steel brakes,still in break in period so havent dont any hard braking. after about 5-10min of driving the car i feel vibration in the steering wheel(shaking) when applying the brakes from 60mph or so.Then after Sometime it goes away and then comes back again. any idea whats happening?

 

whatever it is it is not normal..get it checked out

 

tires out of balance

 

how are tyres out of balance,vibration is only felt while braking??

 

Not the tire. Likely some brake pad deposit on the rotor. You can try bedding the brakes couple times - very brisk braking from 60 mph down to 10 mph w/o engaging the ABS about 8-10 times then cool off the brake by driving around for 2-3 miles. Do not engage the e-brake or step on the brakes when you return home -just let the car 'roll" into the garage and park so you don't get more hot brake pad material on the rotor. You can drive the car normally after this exercise. Do this on a quiet back road so you don't get pull over for suspicion of DUI lol

Hope this take care of the shaking

 

try bedding the brakes. i didnt have your problem but i bedded my brakes anyways back when car was fresh off the boat. if issue still exist, take it to the dealer.

 

Interesting

 

i had this problem before
the lug nuts were loose....

 

got the problem solved,dealer changes the discs,said it was unbalanced from factory.