Sporadic errors - undefined function - class not found - Joomla! Forum - community, help and support

i've been chasing set of problems couple of weeks. errors manifest in few places, i've searched forums far , wide. people similar problems have been answered told overwrite files, i've done few times no success, every calling file , target file. here's examples:

fatal error: call undefined function utf8_ireplace() in [path]/libraries/joomla/string/string.php on line 296
fatal error: class 'jdatabasequeryelement' not found in /[path]/libraries/joomla/database/query.php on line 1133
php fatal error: call undefined function phocagallerybuildroute() in [path]includes/router.php on line 347
php fatal error: call undefined function fpssbuildroute() in [path]/includes/router.php on line 347

the lines fail 2-5 lines after "require_once" or jimport() call file has target function in it. if put code @ line test if function exists, does, doesn't. looks require_once() not loading file. problem is, there no error in log file saying couldn't load file. , doesn't happen time. put code tested function , wrote "found" , "not found" error log , function found not.

it seems error out more in 11:00 12:30 pm local time frame. i've had days 400+ errors in timeframe , not error next 4+ hours!!!

frustrating out. tips appreciated.

joomla 2.5.7, upgraded joomla 2.5.22 , that's when first started seeing problems. created fresh site 2.5.7 , have been buuilding when bam! got first 2 errors in list above.
php 5.3.24

what hosting provider these sporadic errors? server under heavy load?

have compared file class defined same file joomla installation archive particular version of joomla? have heard number of cases file existed size zero, corruption in filesystem or overwriting files. log.php seemed vulnerable this.