I used some custom textured parts for a build a while ago but recently, I saw that there were lines forming on them that looked like grids
Like this:
I have no idea how to fix them, so any tips on how to fix it would be well appreciated! 
There probably is an issue with the texture you decided to use to begin with. Perhaps it has a white line around the picture itself, therefor creating this grid looking texture.
Try making the texture image full screen or covering the whole image like if there are any transparency white and gray lines try to cover it with the image
Oh, and these lines don’t appear in studio as well.