Default [q] no signal after phone times out,

Hi all, I'm a noob to this site and desperately need help.

I've just purchased a S4 I9505 second hand (IMEI checked and not blocked). I'm on the UK 3 network and after putting my sim card in I get signal and reception until the phone goes to sleep (times out) then when I wake it up my reception disappears, with no signal bars, no service and not registered (3) on network. The only way to get reception back is by:

Enabling flight mode then disabling flight mode
Re-enabling my network under "Mobile networks" ---------- > "Network operators"
Re-booting the device

I've already done a "hard" reset in recovery mode and verified the APN and that doesn't help. I've spoken to my network provider whom tell me its a common problem with the S4 which doesn't leave me with any help.

I desperately need your help people.