For this kind situation, it is more likely PHP processes gets into a infinite loop for some reason.
I would just kill those PHP processes, see if it comes back.
Restart LSWS may get those PHP processes killed as well.
If it does come back, try "strace" the process using too much CPU, see if you can find out anything.
CPU cycles used by processes with priority other "0" will be counted towards %ni, your problem is that PHP were using too much CPU, not the process priority.