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MMC Problems!

Hi all,
        I have just completed building a MMC and I am having major problems.
The caps are WIMA MKP-10's 8.2nf 2000vdc/700vac. I am running these as follows.
15 in series=0.00054uf (30,000vdc)
12 strings=0.0065uf   (matched to NST)
15 strings=0.0081uf   (LTR to NST)
Power supply is one 15kv/30ma NST.
Spark gap is variable ,8-12 gaps 25thou each ,vacuum motor quenched(variable).
I have safety gaps+NST protection caps on each HV leg.
All MMC caps have 10 meg 1/2 watt resistors across each cap.Each cap is
soldered to the next with solid copper wire 1mm thick.
The caps are mounted on Perspex with each string bolted to a thick 15mm wide
brass strap. This is mounted in a wooden box with a fan for extra cooling. I
thought that this set-up should be very durable.(WRONG)
As soon as I applied the power 1 or 2 of the caps went Bang.
I checked for poor soldering or cross arcing but nothing, so I replaced the
burnt caps only to have the same problem continue! The funny thing is that some
of the caps have gone bang as much as 1 minute after shutdown, I cannot
understand why if they have bleed resistors across each of them?
Your help with this problem would be most appreciated as I am at a loss.
First Light is so close!
                                           Thanks in advance,