Battery Isolator - doing exactly what it should shouldn't?

I agree the Iso has died and in the most common of ways.
I was warned by a mechanic friend that diode based iso's are problematic. So I bought one that was rated at 160 amps for my 120 amp alt. It still died on me one night with the family on board and tt in tow. I swapped it out for a solenoid operated relay (Napa $25) and never had another problem.