Why does car creep in first gear

Car is an auto trans,for a couple of seconds it feels like car is in neutral then it jumps and the trans failsafe comes on for about 20 seconds and that is when it creeps till it goes into second then it is fine.also it does not do this in the morning when you first drive,it takes till the car is warmed up.

If it were an auto, could just push on the brake pedal to keep it from creeping! Must be manual and trying to still move in 1st gear with the clutch pedal pushed in!