moving clock synthesizer of ad9864 with high temperature

hello support team

we have a problem about clk synthesizer.

when we tested ad9864 in my test room, every thing is ok and don't move the ad9864 clk synth.

but when we go to field test (like as free space and In the vicinity of the sun) after a short time, the clock synthesizer start moving. after shutting own and cooling the system, start it again and in first time the clk synthesizer is ok. whit increase the temperature and system running, start moving again.

all test repeat with ad9864 eval board and  same result achieved.

could you help me about that problem???

regards