The camshaft seal if bad would allow oil into distributor. I think it would probably drip out without accumulating in the cap. There's no cooling passage up that high on the engine. I think you mean moisture gets INSIDE the distributor cap?

Did the problem occur before you replaced the cap - and did replacing the cap do you no good to solve the problem? If so are you buying genuine Bosch caps that match OEM specs? I've had aftermarket ignition parts on various cars that did not pass muster.

You might try the old trick of watching the engine run at night so that you can verify exactly how the ignition is leaking or arcing over.

Do you live or drive in an area with very unusual weather or badly polluted air?