Xperia Ray :: No Service Or Signal?

I'm on o2 uk, I have an xperia Ray with a new sim. My phone constanly loses signal yet when i search or start using the phone even when I haven't moved from where I've been standing or sat for the last hour or so the phone suddenly jumps to 2 or 3 bars on the signal icon! How can that be if I'm stationary it goes from nothing to 2 or 3 bars?? Also my family who are on the same network but have iphones & blackberry don't have this problem, always have strong signals as we live in a good signal strength area but I'm the only one who can't get a signal. If I go to work other people can use their phones-also on o2 but I get nothing! I get texts hours & days after they're sent to me, calls get sent to voicemail as I have no service, o2 sent me a new phone so it's not the phone, it's a new sim, I've tried going on gsm only, rebooting, turning off the radio etc etc but I'm no better off, I'm at my witts end & ready to switch to another brand phone.