AD9957's output have random flashing signal in qduc mode

hi,

I have some problem when i use AD9957,i configure AD9957 in QDUC mode,reference freq is 640M, lvds standard voltage,pdclk rate is configure as 1/2 mode,set i data at rise edge of pdclk and set q data at fall edge of pdclk, set pdclk's polarity inverse,dds's freq is 140M,fpga generate i and q by use dds ip,generate sin and cos, frequency is 1M, sample frequency of i and q is 32M,pdclk is 64M.when temperature is lower, AD9957's output have random flashing signal, spctrum is very bad,this problem is only relative to pdclk's frequency and temperature, when frequency of pdclk is lower, spectrum is good,when frequency of pdclk is high, spectrum is bad.reference source is verg good.