Try mention above on 317:
Boot RouterOS on it, upgrade it to latest version. upgrade system firmware too. Boot SwOS and upgrade it to latest version. Remove TX/RX flow control on all switches.
Cheers
I can confirm that CSS326 working fine with 2.9 and there is still RX overruns.Upgraded two CSS326 from 2.7 and it worked fine. Still having RX overruns.