Why do my caps keep blowing?

Hi all,
I am making my first tesla coil and am having trouble with caps blowing up.
I am making them from 1mm thick poly (10layers of 100um) 
They are vacuumed & filled with fresh transformer oil.
When I place 12kv30ma a.c. across them (0.008uf)  with no load to test them
they puncture through the middle of the poly within 5 minutes.
Perhaps I shouldn't test them like this? 
I clean everything with alcohol and wearing rubber gloves during assembly
then vacuum them for 4 days.
What should I do?
What is the life of one of these caps anyway?
When I have constructed my coil I want maximum life out of the caps so how
long should I set for each coil run?
Thanks in advance for the help.
David Hosking.
email <mailto:davidhosking-at-bigpond-dot-com>davidhosking-at-bigpond-dot-com