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 ≈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 ≈10% of R_n
