Все сайты понизились и когда я перезапускаю Nginx, они работали. Видел, что рабочий процесс отказал в журналах

внезапно все сайты в этом VPS прекратили отвечать. Сервер возрос, и использование памяти нормально. Но видел это в журнале ошибок Nginx:

2014/07/20 07:10:24 [alert] 13671#0: worker process 15020 exited on signal 5 (core dumped)
2014/07/20 07:15:11 [alert] 13671#0: worker process 16327 exited on signal 5 (core dumped)
2014/07/20 07:15:11 [alert] 13671#0: worker process 16036 exited on signal 5 (core dumped)
2014/07/20 07:15:11 [alert] 13671#0: worker process 16369 exited on signal 5 (core dumped)
2014/07/20 07:15:17 [alert] 17646#0: [ngx_pagespeed 1.7.30.1-3609] [0720/071517:FATAL:critical_selector_finder.cc(118)] Check failed: driver->property_page() != NULL.
Backtrace:
    nginx: worker process() [0x4c6c23]
    nginx: worker process() [0x4c3dc3]
    nginx: worker process() [0x850851]
    nginx: worker process() [0x850e57]
    nginx: worker process() [0x84bd63]
    nginx: worker process() [0x5270fa]
    nginx: worker process() [0x539953]
    nginx: worker process() [0x6977fd]
    nginx: worker process() [0x52d239]
    nginx: worker process() [0x5051ad]
    nginx: worker process() [0x501b50]
    nginx: worker process() [0x508446]
    nginx: worker process() [0x655772]
    nginx: worker process() [0x6e97dd]
    nginx: worker process() [0x6ec0e7]
    nginx: worker process() [0x655772]
    nginx: worker process() [0x6f546b]
    nginx: worker process() [0x6f78cc]
    /lib/x86_64-linux-gnu/libpthread.so.0(+0x7e9a) [0x7faee2be0e9a]
    /lib/x86_64-linux-gnu/libc.so.6(clone+0x6d) [0x7faee03e43fd]

Backtrace:
    nginx: worker process() [0x4c6c23]
    nginx: worker process() [0x4ace7e]
    nginx: worker process() [0x4c3e9f]
    nginx: worker process() [0x850851]
    nginx: worker process() [0x850e57]
    nginx: worker process() [0x84bd63]
    nginx: worker process() [0x5270fa]
    nginx: worker process() [0x539953]
    nginx: worker process() [0x6977fd]
    nginx: worker process() [0x52d239]
    nginx: worker process() [0x5051ad]
    nginx: worker process() [0x501b50]
    nginx: worker process() [0x508446]
    nginx: worker process() [0x655772]
    nginx: worker process() [0x6e97dd]
    nginx: worker process() [0x6ec0e7]
    nginx: worker process() [0x655772]
    nginx: worker process() [0x6f546b]
    nginx: worker process() [0x6f78cc]
    /lib/x86_64-linux-gnu/libpthread.so.0(+0x7e9a) [0x7faee2be0e9a]
    /lib/x86_64-linux-gnu/libc.so.6(clone+0x6d) [0x7faee03e43fd]
2014/07/20 07:15:17 [alert] 17645#0: worker process 17646 exited on signal 5 (core dumped)

Какова могла бы быть проблема? Как предотвратить его снова?