New IPhone 4 lagging on games

I bought an iphone 4 three days ago and it worked perfectly fine with games until yesterday.
every time i would play a game like temple run or nazi zombies, there's like a millisecond lag every 4 seconds or so. I mean, this isn't i big deal but you would expect it to run smoothly for the price you pay. I even tried it with my dads and it works fine.
I have abbout 5gb of stuff on my phone which isn't too much.
However, whenever i put it into flight mode, the games work perfectly fine. Yesterday and today, i'm at my grans house where there's a very bad connection. You either have no signal or 1 bar. Could this have something to do with it? Because it started when i got here. Also, when i turn it off and back on, they would run fine but gradually start to lag more after time.
please help, because little things like this really piss me off!

Try checking if all the unnecessary apps are off and it is not causing more lag also check if your GPS is off that also causes lag.

Go on apple.com and call them. Same thing happened with my mum

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