automated install with kickstart file and vmware 6.7 doesn't work, but sometimes it does

I have a relatively simple kickstart file that works always with 6.5

and only "sometimes"  with 6.7.

When it doesn't work, it hasn't executed anything after the %firstboot section.

kickstart.log file is empty in either case (success or not)

weird thing is I have 2 types of install, with 10Gbit nics and with 1Gbit nics.  The 10Gbit nics version works nearly always and the 1Gbit nics version works nearly never.

But in both cases it sometimes works and sometimes not.

I already increased the sleep value but that doesn't change anything.

it's the typical kickstart file

#THE BELOW PART ALWAYS WORKS

#=============================

vmaccepteula

clearpart --firstdisk --overwritevmfs

install --firstdisk --overwritevmfs

rootpw blablabla

network --bootproto=static ---- .... with all optoins

reboot

#THE BELOW PART "SOMETIMES" WORKS

#==================================

%firstboot --interpreter=busybox

sleep 30

#do esxcli stuff...

if only I could find some logs to search through.