My amplifier was working properly when in use for a couple hours each day for a week, however today after 3 hours of proper, and stable, operation, the amplifier stopped working may be damaged. The current drawn now rises quickly and reaches the 1.3 A spec after the drain voltage reaches 0.7 V, with the gate voltage of -1V. When it was working properly, the gate voltage of -0.2V would yield the correct 1.3 A current draw, with a 7 V drain voltage when following the proper biasing procedure. I was monitoring chip temperature during these hours, and it was maintained below 40 deg C (measured with thermocouple) using a cooling system comprised of TECs, fans, and a heat sink. This is much below the 85 deg C max rating. My input signal was ~ 8dBm (sine wave at 10 GHz), which is below the max input power as well. There doesn't appear to be damage to the chip or the capacitors on the board (no bulges or deformities).
Do you have any recommendations as to what to test to see what may have gone wrong this time? Is this amplifier not meant to work over the course of hours?