I have the same problem with my g300.
2 weeks ago i found it dead and sent it to service the same day. They just told me yesterday that the mainboard needs to be changed.
Did they found out your phone was rooted and there was no problem with the warranty or they didn't found out? I'm asking because my phone was rooted also and I'm a bit nervous about this.
Suprised taken 2 weeks mine was back in 4 working day's but that was going into the vodafone store and them sending it off.
I sent it off rooted there was no mention about it at all.
I think they test the board replace it then just do a fresh install, I would not worry about it being rooted.
I would say we both have the same fault.
You should also get a new serial number and IMEI number as well as I founf out today when doing stuff.
Edited by ezdub, 18 June 2013 - 10:52 AM.