We introduce the Quantum Existence Discontinuity hypothesis (HDEQ, formerly QTDH/HDTQ), positing that the active existence of quantum systems is intermittent — within a continuous time — in cycles of duration τ₀ with an active fraction ε. Three results: (i) global unitarity is preserved by construction via a temporal projection operator (Theorem 1); (ii) a phase-stability criterion selects the preferred basis as the eigenstates of the effective Hamiltonian (Proposition 1); (iii) an irreducible intrinsic-decoherence floor is predicted under a dispersion hypothesis. An intermittent Fermi golden rule gives Γ_HDEQ = ε² Γ_QED, yielding the first direct experimental constraint ε > 0.98 (¹⁷¹Yb⁺). This version (v6) renames the framework and clarifies its ontology: time is a continuous parameter; it is the active existence that is intermittent. The framework does not claim to solve the measurement problem in its strong form.