Hello,
Note that the AD9240 was released back in 1998. This is the 1st time I heard of any temp related stability issue with the reference buffer using recommended decoupling circuit shown on datasheet with good layout.
The datasheet states that the AD9240 reference buffer need to be decoupled for both stability and ac performance issues. The 0.1uF should be ceramic multiilayer NPO surface-mount type and positioned close to the CAPT/CAPB pins. The 10 uF capacitor (a tantelum capacitor with + terminal connected to the CAPT pin) can be located further from the IC due to its larger physical size. The NPO temperature stability should be suitable for decoupling to -40 C,
Regards.