  We study the mechanism of instability in transition edge sensor (TES)
bolometers used for ground based observations of the Cosmic Microwave
Background (CMB) at 270GHz. The instability limits the range of useful
operating resistances of the TES down to $\approx$ 50% of $R_n$, and due to
variations in detector properties and optical loading within a column of
multiplexed detectors, limits the effective on sky yield. Using measurements of
the electrical impedance of the detectors, we show the instability is due to
the increased bolometer leg $G$ for higher-frequency detection inducing
decoupling of the palladium-gold heat capacity from the thermistor. We
demonstrate experimentally that the limiting thermal resistance is due to the
small cross sectional area of the silicon nitride bolometer island, and so is
easily fixed by layering palladium-gold over an oxide protected TES. The
resulting detectors can be biased down to a resistance $\approx$10% of $R_n$.
