Hi,
mine does exactly the same, it occurs most when the phone is charging, i hear a message notification so unlock the screen to see who has messaged and the phone re boots, this can happen three or four times before i get to read the message. un plugging the charger helps but it still does it ocassionally when not charging. its really annoying when you want to phone someone, pull it out your pocket, unlock it and it just keeps re booting everytime you unlock it.
i removed the gmail widget and i thought that was it, then a few hours later back to the same. i tried wiping the phone back to factory so that no apps on it but still the same, i have now turned the screen lock off and it never shuts down.
also i contacted LG and im pretty shocked at their attitude. first they said they hadnt heard of this problem, then the guy admitted yeah there were some handsets doing this.
i asked if i could get it repaired, he said i need to go to uk where i bought the phone at christmas and i could get it looked at in dubai where i live but i would have to pay for it.
serious its a great phone but i think it will be my first and last LG phone.