ADC I/Q imbalance

We work with the EVB using the no os software.

We have a problem: it seems that some times after configuration everything is OK, but many times ADC I/Q mismatch is very poor: we transmit CW at 800k from the center and we observe very nice sine waves for I and Q - but they are not equal in power. In fact, they are very different - one is 8-10 db higher than the other. 
Any idea what might cause this?

Thank you,

Sorin