Noise pattern in STL-6303

Hi,

We have a STL-6303 for more than a decade and in the past few weeks we noticed a very noisy structure in the Bias.

For example on the night of January 4th is a good bias with STD of about 10:


On the night of January 30 the bias is bad with STD of about 70:

Since then we are getting bad bias frames with that noise pattern all the time.

It seems that the problem increased between the above dates since on January 10 the STD was about 15 and on January 10 the STD was 20. We do not have Bias images from all dates since weather was bad in most of the nights during these two weeks. The average bias level is the same of about 1000, but the STD seems to increased in a mater of about two weeks.
This noise is affecting our images badly to the point we cannot do any reliable science with them.

Can anyone suggest any explanation and/or resolution for that problem.

Thanks,
Shai