MMC partial failiure

Hi All,
         my little coil's mmc seems to be undergoing some sort of weird 
failiure mode.
The coil functions normally, throwing nice 18" arcs and drawing 80ma -at- 5kV, 
then it starts to pulse in and out of tune.  
The secondary discharge stops almost totally and the spark gap makes the sort 
of hiss that is does when the coil is very badly tuned.  After a few seconds 
it pulses back into tune only to drop out again and repeat what I have 
already described.
My theory is that in one of the 3 strings of caps is a cap close to death and 
that it is breaking down reducing the capacitance of the string and taking 
the coil out of tune, then self healing a bit then breaking down again.  
Unfortunately not having an accurate C meter I can't measure the strings to 
check this.  
The caps are 1600V 47nF philips 378 series in strings of 10 with 3 strings in 
parallel  and the neon is a 6/60 (I said 80ma 5kV because it sags quite a lot 
under load) 
It is not a reso-cap so I don't see how a large enough voltage to damage the 
caps is being generated off a 6/60 neon?

Semi-baffled in GB
Nick Field