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HMC637LP5 Output Match Question

I am having a repeated failure with the HMC637LP5E that is proving tricky to isolate. I think that the cause could be instability at the output side.

The amplifier is used from 20MHz to 500MHz in this application. On the input side I have a small value attenuator followed by the DC block. The output has the Bias tee and DC block, followed by a lowpass structure matched to 50 ohms. This outputs into a TNC connector feeding a ~5' cable.  My question is, is there a particular output load impedance that will damage this part or cause it to run away? When I see the failure, there is no degradation in performance; it is a very quick catastrophic event that I have been unable to capture.

My thinking is that when this cable is left unterminated, or as it is being connected to a 50 ohm load, some reflection is occurring. Ideally I would run through the load-pull test, but I am finding it tough to locate a trombone that goes down all the way to 20 MHz.

Over-driving at the input is very unlikely. I am very sure that I am sequencing the part properly. The failure does not occur at turn-on or turn-off; I have seen it happen in 4 amplifiers after a few hours of constant operation at temperatures well within the operating limits.

One final note for clarity, this is not the HMC637ALP5E part. I am using stock of the now-obsolete HMC637LP5E.

Thank you