HELP! Momentary Electrical Shutdown Stops Engine Dead

It's a '95 900 Turbo Convertible, and I love it even so. Sometimes, when you put a surge load on the electrical system, as by applying brakes, pressing the electric window button or hitting the A/C, the system voltage either drops severely or momentarily cuts out completely -- for a split second only, but enough to shut the engine down. Confoundingly, the problem is intermittent. I put a new battery in, and the problem went away, but returned a month later. It's got a brand new Delco alternator, which maintains 14 volts with a mild load and drops to about 13.6 with lights and all accessories running. When I clean every ground and hot connection I can locate, including the battery terminals, the problem sometimes ceases for week or so and returns. This, despite the fact that I've run a "sister" cable from battery ground to the engine, and have cleaned the battery ground-to-chassis connection more times than I can remember. Is it possible that there's a faulty circuit board that intermittently goes fritzy when the voltage drops? Anybody?