High system cpu load (%sys), system locks

I know this thread is a really old and I know you are already aware of this, %sys --> if the cycle is spent in %system then much of the execution is done in lower level code i.e might be issue on kernel side. If this issue is reproducible again, please collect the output of:

echo t > /proc/sysrq-trigger

And check system messages (var/log/messages or /var/log/syslog) to see if any thread may be using a lot of system CPU time.