Display working but touch does not

Hello everybody.

First I'm sorry for my english, I'm from Czech Republic so it not will be great.

I have this problem with my iPhone 4 with iOS 7. When I'm working with it, sometimes happen that when I'm touching the screen, it's not responding. Display is working, because I can quit running application by home button but it's not responding on touch. Sometimes it's not working for only 5 sec., sometimes it takes 10 sec. and sometimes I must lock the iPhone, unlock it and it works again, but sometimes I must lock it, press lock button again and again for 3 times or more and after that touch is working again.

I had already try to open the iPhone, check the "flex cabels" if they are OK, unplug them, plug them again but I'm still having this problem.

I don't want to buy a new display and risk that it will not help.

Any suggestions what can cause this problem?

Thank you for you help