Touch problems multiple iPhones

Hello guys,

I own a repair shop and i have a verry bad month of having the same problems over and over with iphone 6 - 7 -8

i recently refurbished 2devices and these phones have problems with the touch.

also with phones that only the battery have been changed have this problem.

all phones have 12.1 update

every time the iphone is booted after a software reset the touch freezes a lot of the times, sometimes i have to open the phone and reconnect the battery to make the touch work aggain.

today a client came to buy a iPhone 6 and it has the same problem.

the phone works normaly and i tested all so did a factory reset.

After the iPhone was starting and i insert the sim card the phone freezes…

i have remove the sim card and do a soft reset to get the touch back to work.

phones that i have the same problem are as followed.

iPhone 6 2X Original screen

iPhone 7 1 X Full refurbished ( sometimes evens heats up. )

iPhone 8 1 X New screen, sometimes my touch does not work at all..

i checked all connectors to make sure that they are 100% fitted but i cant find the problem!

Its messing me up and gives me a bad feeling.

Is it a possibity that this is IOS 12.1.1 problem? other wise i dont know…

i never had so much troubles in the past , only since the new updates i get a lot of problems so my opinion is… that the only thing i can think of is software related.