1px rainbow artboard border wont go away

This has been happening for about 6 months and i've been ignoring it. Please can anyone help it is driving me mad and causing problems.

at various zooms the art board looks like this

1 px border around edge in random coloured pixels. This is definitely some sort of glitch

Am i doing something wrong?

Note the image is weird-looking (grey rectangle over image) because it's for a client so i can't show the image but hopefully you can make out what is going on here