I had the exact same problem. I abandoned that server and built a new one utilizing the full blown MS SQL 2008R2 and the stability issue went away.

I updated the avast policies in the old avast console to point the client machines to the new avast server and scheduled a reboot of the old avast server every half hour and eventually the computers got the policy and migrated over. Once the last one migrated over, I just destroyed the old VM.

I think it’s SQL Express crapping out and not so much Avast.