Iphone 4 lagging with 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, doodle jump and nazi zombies, there would be a millisecond lag very 4 seconds or so. I mean, this isnt a big deal but you would expect it to run smoothly for the price you pay.

I even tried with my dads and brothers and theirs work perfectly fine. I only have about 5gb of stuff on my phone which isnt a lot really. and definitely shouldnt slow my phone down.

However, whenever i put my phone into flight mode, the games work fine. Yesterday and now, i am at my grans house where the connection is very bad. You either have no service or 1 bar. Has this got anything to do with it? Also, whenever i turn my phone off and on, the games would run fine for about 20 mins but gradually start to lag again.

Please halp, because little thing like this really get on my nerves!

thanks!