Feeling good about yourself is not the safe end of this. The high and the low come from the same borrowed source, which means the high can be taken back at any point.

What the research says

Across four studies, rage, hostility and aggressive behaviour were driven by narcissistic vulnerability rather than grandiosity, suggesting explosive anger tends to sit on a fragile self-view rather than an inflated one. That fits the idea that the shakiness matters more than the height. The work combined correlational and experimental designs in mostly non-clinical samples, so the pattern is suggestive rather than settled.

Strength of evidence: one line of research. A single study, or a theoretical account.