the two bolt head gap?
That will definitely have thermal issues..
I do not use a fan to cool it down either.
I was wondering if you knew any reason why the spark gap was dieing away?
what happens is that you get a hot spot on the electrode, and that
causes the gap to fire earlier,at a lower voltage, when there's less
energy stored in the primary capacitor, so the resulting secondary
sparks are shorter.
You need a fairly healthy air flow for a bolt head type gap. I'd get a
couple pieces of copper pipe and hook those on the bolts and use that. A
short piece of pipe has a lot of surface area, and is gently curved,
both of which help spread the sparks around.
A low speed muffin fan to gently blow air across it helps too.