Touch screen works briefly then freezes
I have an iPhone 6 and the touchscreen will freeze randomly. Sometimes I have to wait five minutes from start up for it to respond. Sometimes it works straight away but only for a few minutes. Sometimes it'll work for twenty minutes. There is no rhyme or reason to it freezing.
Here's what I've tried,
1. Soft resets - didn't help.
2. Factory reset and set up as new device- didn't help
3. Fitting x 3 new known working screens - didn't help
4. Cleaning the connectors with iso alcohol - didnt help
5. Updating software which went fine (no error codes) - didn't help.
Probably helps to mention I've switched the motherboard out of a damaged housing to another genuine housing. I own a small repair business and can strip most parts almost blindfolded.
I can't see any physical damage to the board and if the mobo touch connector were damaged the rogue white lines would appear.
Any advice would be appreciated.
Thanks in advance.