My SVX does not start

Lately I sometimes cannot start my 1992 SVX. At first, I thought it was the battery, since it was in the car when I bought it, about a year ago. I was able to jump-start the car with no trouble. I replaced the battery with a Sear Die Hard Gold and had the Sears technician test for charge and leakage. All tested OK. Three days later the car would not start. I again used my trusty jumper cables abd brought it in to the dealer. They tested everything and could find nothing wrong. The next time it happened I connected a battery charger. The meter (and the charging time) indicated the battery was substantially discharged. It had enough power to light the lights, open the doors, etc. but not enough to start the car. It took about 3 hours to fully charge the battery (at 12 Amps.). The car then started with no problems.
I noticed that the problem usually appears after heavy rains.

Is it an electric short? A starter drawing more current than usual? Something else? How can I find out?

I hate to bring it to a dealer a gain and be told there is nothing wrong with it.