Unconventional IRF shape

[jpfeifer]

See

Issues with Ramsey Planner Model (Blanchard Kahn with undetected issues in model diagnostics)

Sorry for the delay. But I don’t have anything to say beyond the generic “Try working with a simplified version of the model”. Oscillations like you describe are typically a sign of a complex eigenvalue. They are most commonly caused by either a timing error or a problematic parameterization in something like a (fiscal) feedback rule.