Screen randomly goes black, comes back on after random amount of time.

Problem began about a few weeks ago. Was checking text messages and screen went black. Reset had no effect. After leaving it alone for a few minutes, it came back on. Phone does this periodically. Sometimes it'll stay black for a few minutes, other occasions for hours. On one occassion for a couple days. Everything else works fine. Siri, sounds, locking/unlocking,etc. Soft and hard resets do not work when it goes through these spells.

Had it tested with another screen during one of these occurences but same deal. Worried there's an issue with logic board. iOS diagnostics point to battery requiring service but not convinced that's the main issue, though it is a possibility I guess. Battery still holds charges well. Thinking it is a circuit issue. If I look at the screen in glare off sunlight, I can make out icons and images. What could be causing these "black-outs?"