Why does my cellular signal drop on my iPhone 5?

I have jailbroken my iPhone 5 and it was working perfectly fine until now. I leave my iPhone 5 on idle (Stand by) for about one hour, when I come back to it I seem to find that my iPhone 5 cellular network has no signal for 10 seconds and then finds it. I know it's only 10 seconds but for a £500+ phone and a trustworthy network I should be able to receive a good standard of service. The network is '3, three' and it is based in the UK.

Two reasons:-
1st - Its an iPhone
2nd Its on 3 network