MSP430F427A SD16 ADC sampling issue

I am using MSP430F427A with SD16 ADC for single phase energy meter application.

When I set the SD16 for higher sampling rate (4Mhz) then the sample values gradually decrease even if the input is kept unchanged, and sets down after few minutes to a constant value. But if I decrease the sampling rate (to say 1MHz), the sample values are stable.

I am new to the MSP430 world, please suggest any solution.

Thanks in advance.