In PHPBB> CP> General > load settings > Please check options "
Limit system load:
If the system’s 1-minute load average exceeds this value the board will automatically go offline. A value of 1.0 equals ~100% utilisation of one processor. This only functions on UNIX based servers and where this information is accessible. The value here resets itself to 0 if phpBB was unable to get the load limit.
Maybe you have set it too low, so you are hitting limit.