Touchscreen only works in low brightness!?

Hello all

Strange problem:

For the past 2 days my iphone 5 only works for 5 seconds upon unlock-lock. After that the touchscreen becomes unresponsive (i.e. ghost touches) and then does not work at all no matter what part of the screen you touch. Note that the hard buttons (volume, home etc. still work).

Then, I have to wait around 1-2 minutes for it work for 5 seconds again.

I have updated to ios 8.1, reset all settings, erased all content and setting and STILL the same problem.

Today however, it occurred to me to lower the brightness within the 5 second time frame AND IT WORKED! BUT only for 3 minutes, and with a few lags without any applications open. After that I opened safari, passport and messages and the touchscreen was, again, unresponsive.

What can the problem be?

Is it overheating? (although there are no signs of this i.e. not warm/too hot.)

Ken (not related to Ryu)