If you have the 1' toe out set properly on each side with a setup station, then the car should go straight. If sometimes it does not, than you either hit something and need to realign it. Or there is too much slop in the steering/C-hubs/arms/rear uprights. You can use the shim set SW03/V2 to reduce the play to an acceptable level. However it is quite tedious and you have to be very careful not to cause any binding in the parts.
Beside the chassis check also your body if everything is centered, especially the rear wing.