hmc834 temperature problem?

We are using the HMC834 in a radar app as a second LO. The output frequency is set between 7.6 GHz and 8 GHz in pure integer mode using a 95 MHz reference frequency and a 19 MHz PD frequency. Loop filter bw: 130 kHz. From temperature chamber measurements the PLL goes out of lock under 8°C and seems to get back in lock under 0°C. It should work from -30°C to 75°C. There are many registers and some bitfields without explanation. We init the chip as:

        pll_write_vco(1, 0x1F);
        pll_write_vco(2, 0x1C1);
        pll_write_vco(3, 0x0);
        pll_write_vco(4, 0x16D);
        pll_write_vco(5, 0x0FC);
        pll_write_vco(6, 0x0FF);

however I am not sure if autocal runs correctly and if this device goes out of lock by temperature change, and what to do then. What if the PLL goes out of LOCK? just rewrite the N register. (We do this now, but without success). What is the correct sequence for programming the PLL. Are the provided values OK? At the moment we can not find any cause why the PLL goes out of lock. Reference signal is 2Vpp clean sinus, clean spectrum. (All other PLLs run on this clock and are OK) Supply voltages are fine.What can be the cause?

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