Still sounds like a bad connection. It seem the starter circuit allows current to flow under light load but the circuit opens when heavy draw takes place. Typically a loose connection causes this. I've also seen defective batteries act this way. When you had the battery tested, did they perform a load test on it with a carbon pile. Load the battery up with what your starter typically draws and see what the voltage drops to (shouldn't drop below 9.6 volts) or if the battery shorts out internally causing a zero voltage for a moment indicating an internal short in the battery.