[Amps] Vacuum variable hipot

[Steve]

I have a Jennings UCSL1000 - 1000pF, 5kV cap. If I put 5kV dc across it 
and run it through its tuning range, the leakage current is sometimes 
zero but fluctuates and there are places where the current leaps up to 
50uA. At those points the current falls to zero if the voltage is 
reduced to around 4kV, although I have heard a couple of arc 'pops' from 
it sitting there at 4kV.

At other settings where the current is lower, 5uA or less, the leakage 
dies away to zero if I leave it sitting there for a while.

Is this normal behaviour? Is this one a duffer?

Thanks,

Steve