I would have to say bad part or an improperly seated digitizer cable. I'm assuming if you have worked on these before...then you checked that to make sure it was seated properly. I've fixed over 1,000 iPhone 4 and 4s screens and a lot of issues can be blamed on poor quality parts. Generally, if you do a bunch of them its best practice to find a good supplier has high quality parts with OEM LCD's in them and keep ordering from the same person.
If its the touch portion that is "freaking out" and touching on its own....Then I would highly doubt its anything to do with the prox flex. Unless things are getting touched when the phone is being used for a call....then the screen is probably not shutting off when put up to ear to talk...and cheek or face is pressing the screen. This is a prox issue. But if you are just holding it in hand and its doing its own thing...shouldnt have anything to do with the prox.
Hope this helps.