My iPhone 5s touchscreen stopped working

My iPhone touchscreen recently stopped working. Nothing happened: I was using it and suddenly it wasn't working anymore. Sometimes the touchscreen works, but only for a few minutes. Also, there is a line of white squares on the lower part of the screen.

I've tried hard resetting it more than once, but it didn't work. I also did a factory reset, which solved the problem, but the touchscreen stopped working again.