Hi KarVi,
my personal suggestion is to first figure out whether the entire SD-card is error free and works as expected:
- Make an image backup of the sd-card with a Windows PC and Win32DiskImager
- Follow the two steps in PI 3 b osmc boot error - #9 by JimKnopf post 9; report here the average write and read speed after the test. I know it sounds very time-consuming but while the h2testw writes to the card watch for the write speed whether there are hard speed drops
- if the card is entirely error-free and shows normal average speed behavior, restore the image backup
With that we should have understood for sure whether the card is (part of) the problem or not.
Keep my fingers crossed for you that it is only a malfunctioning card.