When you flip the pto switch...does it die normally like you turned off the key, or does it squeal or smoke?
If it the mower starts, and lets you drive it around until you turn the pto on, I would look at the pto clutch, or pto switch, pto safety switch, or wiring to them.
Maybe they're better now, but it didn't used to be too unusual to get a new clutch that was bad out of the box.