I know I am replying 3 years after this question was submitted, but I had the same issue 3 days ago and lost a lot of time trying to find a pattern or a software cause. Then I decided to open up my Lumia and see if I can spot some loose cable. I found the the touchscreen cable got unstuck when i touched it lightly.
It took no more than one hour to open, fix the cable firmly in place and assemble it back. There are many good youtube videos that explain how to open your Lumia and which cable does what.
I urge anyone with same problem not to lose a lot of time restarting or formatting the phone, if the phone randomly becomes unresponsive to touch then it probably is a hardware fault.