ADC78H90　conversion problem at 7.1M clock

Hi all,

My customer reported that ADC78H90 can not convert an input dc signal correctly when changing the input channel from AIN1 to AIN5. A5's voltage level is 0V. Moreover, an input stage has a voltage follower circuit. 
SCLK is 7.1MHz.

The test result are as follows;

AIN1 -> AIN2 OK
AIN1 -> AIN3 OK
AIN1 -> AIN4 OK
AIN1 -> AIN5 NG
AIN1 -> AIN6 NG
AIN1 -> AIN7 NG
AIN1 -> AIN8 OK
AIN2 -> AIN5 NG
AIN2 -> AIN6 NG
AIN2 -> AIN7 NG
AIN5 -> AIN5 OK
AIN6 -> AIN6 OK
AIN7 -> AIN7 OK

Before changing Ch, ADC can work correctly.
Below is the result of AIN5( 0V) after changing input channel from AIN1 to AIN5.
The conversion data was settled at around 0x05h when continuing to convert. 

AIN5 input cuicuit:

According to the datasheet, after changing an input channel, the converted data is outputted in the following cycle.
Moreover, the input stage can be considered to sufficient low impedance for an ADC input.

Why there is the difference result between channels?

Is there any solution for this? 

Regards,
Toshi