There is bit of a discussion going around on various forums about this problem.
It seems that this non-responsiveness mainly occurs when you take the phone out of your trouser pocket (well after its been in the pocket for a while)
Some believe it happens as result of phone screen becoming hypersensitive due to friction.
One of the solution is, when you are putting your phone in the trouser pocket, make sure you place the screen to face outwards, (not to face towards your body)
Try it and see how it go.