LMX2615-SP: Problems with locking and PLL getting "lost"

Using the device with a 100 MHz reference source at the low end for input of about -4 dBm. Occasionally, about 10% of the time, commanding the device to a new frequency (1 of 5 different ones) fails and the device fails to lock and is at more or less a random frequency. Bit rate into the SPI is about 1.7 Mbps. We have to basically reset things by writing to register R0 to get the hangup to clear itself. Has anyone else seen this behavior? We're seeing it on multiple devices.