Streaks on the screen would either be the display itself is going bad, or the cable that connects it to the motherboard is damaged, bent, etc., and needs replacing.
While for some things testing it on an external monitor would help, for this not so much.
If you feel comfortable opening it up yourself (and it isn't under warranty) then check the cable that connects the monitor first for any problems. If you find none, try replacing the cable (it is cheaper). Should that not resolve it, then you are talking replacing the display.
Now if it is under warranty, just contact the manufacturer about a repair/replacement. If it isn't, and if you don't feel comfortable doing the work yourself, then try a local tech.