Do you notice the tach drop to zero before the engine quits, or does the engine die and the tach drops with it? Start with pulling any codes that might be present.
The key is to puzzle out of this is electrical or fuel. It doesn't sound like an intake leak so the only other thinks that could stop the engine would be loss of spark or loss of fuel pressure.
If the tach drops first, then the engine, I would say there is an electrical problem somewhere.
but if that isn't what happens, then it is still either electrical or fuel. You should think about picking up a diagnostic fuel pressure gauge. Hook it up on the fuel rail and see if you can see the engine quit and monitor fuel pressure. Fuel pressure should maintain even if the engine quits. The fuel pump should maintain pressure as long as the ignition is turned on.
If fuel pressure drops, you could have a failing regulator, or a wiring problem with the fuel pump, or a failed fuel pump. Or maybe even a plugged fuel filter.
Has this only happened at idle? Never while going down the road have you felt a hic-up in the power delivery or noticed the engine try to die?
Electrical problem could be more complex. It could be anything from a bad battery connection to the harmonic balancer failing.