Hi guys ive got a problem with my car, sometimes when you start the car it will turn on then it will turn straight off. I think it might be the ecu any others ideas?

could be the imobliser??

Battery.

I dont think its the battery as i still have power

change the battery, had the same problem

what happens when you apply a bit of throttle as/after you start it? does it still stall?

if not, i'd put money on base idle.

it doesnt stall when i put my foot on the pedal

Doesn't matter if the battery has enough power to light up headlights, or just crank motor over.. it needs to be in excellent condition to keep the magna running after start up.
I'll bet the problem is in the battery ( being weak or having a bad cell ) or the key ring antenna is faulty. UTFSE!