wp-db.php and pluggable.php error

I am hoping that anyone out there can please help me with this. My hosting company recently suffered a catastrophic failure of its disk subsystem. I was forced to upload an old backup …all well and fine but from time to time I get these errors.

Warning: mysqli_set_charset(): Error executing query in /webroot/y/e/yesbitch001/sitename.net/www/wp-includes/wp-db.php on line 818

Warning: Cannot modify header information – headers already sent by (output started at /webroot/y/e/yesbitch001/sitename.net/www/wp-includes/wp-db.php:818) in /webroot/y/e/yesbitch001/sitename.net/www/wp-includes/pluggable.php on line 1174

I re-uploaded wp-db.php and pluggable.php from a fresh copy of wordpress all good …but those errors keep appear from time to time.

thanks in advance!