Because of the big size the whole LCD assembly is being stressed by bending and flexing rather easily compared to other phones. That leads to the well known ghost-touches, shaking screen and sometimes vertical or horizontal lines showing up on the screen.
ONLY way to fix this is by replacing the whole assembly (LCD, digitizer and frame).
Don't bother with cheap parts off ebay - they have a shabby quality and not fit will plus the glas tends to crack easily. A genuine Nokia assembly from a trusted seller is around 200 Euro. Regarding the age of these phones it is also wise to repalce the battery while the phone is torn down. Originaly battery costs around 40 Euro.
Although I did these repairs for two of my friends phones, I not consider it worth spending so much and sold mine as defective on eb*y. Better go with a new phone - and with the latest events regarding W10m I would not advise to keep riding a dead horse...