I have been experiencing UCC3813-5 failures in a sepic dc/dc converter so I decided to power the UCC3813 with an 8 volt power supply (through a 250 ohm resistor). The loop was opened so current sense and feedback inputs are essentially grounded. The output is a 180kHz squarewave, as expected. After about 2 minutes the REF output starts creeping from 4v to about 7v and the oscillator stops and the output locks high or low. After a 5 minute rest the same behavior will repeat. What could cause several of these parts to behave like this? I am careful about static and soldering procedures and am using a good bench power supply. I am also running out of UCC3813s.