Could be a power issue. What he describes as lines on the screen sometimes, could be power that dies after a split second. Might for example be related to the power regulator board in the lower left corner, for example bad capacitors. It could be due to dirty/loose contacts on the power switch, corrosion in the DC jack. (It has an internal switch that disconnects the battery when a DC jack is inserted, and that internal switch could be broken.) But hard to tell without further information. Likely to be fixable, but you need to find the problem.
Would help to see a video of the issue once you receive it, as well as pictures of the internals.
I have literally thousands of components, Including caps regulators, ECT. I salvage electronic components for a hobby with a desoldering iron. So if its a power issue, hopefully, It should be an easy fix. Worst case scenario, its the typical unifixable horizontal lines. But based on the description, It most likely isn't. Which is good.