Kobo Aura bold text all of a sudden...

Hello all,
Sorry if this has been asked before but I had trouble finding the same problem through forums.

Anyway, as the title says sometimes all text becomes bold.
I'm reading MOBI book and everything seemed perfectly fine.

After I finished reading I switched it off, and when I turned it back on the last page I was reading, all of a sudden all text became bold.

I switched it off for about an hour and when I turned it on it all changed back to normal font.

So, the bold thing comes and goes and I hate waiting an hour or so just so I can start reading again.

Any help/fix would be appreciated,
Thanks.