[Milsurplus] OK Smart People: When is a Cap NOT a Cap?

[David Stinson]

As I wrote earlier, I'm working on the GF transmitter tuning units.
Getting them to load a 50-ohm antenna requires a series capacitor.
The 75-meter unit uses 150 pFd.  Initially, I dug a couple of micas
out of the junkbox and they worked, delivering 12 watts to the
dummy load.  A junkbox 150 pFd ceramic worked as well.
But I had no idea of the voltage rating of these caps or their
long-term prospects.
So I went to Digikey and searched for some
150 pFd, 2KV ceramic caps.  I settled on:

http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&KeyWords=490-4271-ND

Got them in and installed one.
The max power I can get out is about 5 Watts.
The cap heats, too.  It's eating power.
The whole bunch is useless.

Tell me what I did wrong.
When is a 150 pFd, 2 KV ceramic capacitor.... not one?
And can your recommend a better solution that isn't $20 a pop?

73 DE Dave AB5S