Screen keeps going dark, runs slow

Hi all,

Screen suddenly went dark while computer was in use.

Backlight and keyboard light are working fine.

Reset NVRAM and SMC, no luck.

Reseated LCD cable on logic board side.

Hooked up to external monitor, all seems fine through external display.

Display came back slowly after cable reseating (going to patchy white, then suddenly all there), and for a few days would sometimes revert to patchiness before coming back.

Worked fine for a bit, but going dark intermittently - a restart or sleep would usually fix it. But computer seemed a bit slower.

Now (a few weeks since the problem started, and with everyday use) screen stays dark for about 5 minutes after sleep, and computer is noticeably a lot slower.

Was convinced (wishful thinking?) with help from this forum that this was a display / cable issue, but now not so sure.

I live remote and techs are hard to get to - is there any way I can test if this is a logic board issue? or any other ideas? I can send it off for a logic board repair if needs be, but just keen to diagnose in case its something simpler (and cheaper!) that I'm missing.

(I actually posted as a display question earlier, but the problem seems to have evolved so thought it warranted a new question rather than an update? please correct me if I'm wrong)

Thanks in advance!