The source address persistence profile assigned to both the EWS and RPC virtual servers should persist requests for those services to the same CAS for the same client IP.
How long does it take for EWS to stop working? When it does, do you see EWS connections going to a different pool member than the RPC connections? If you run Fiddler on the client or tcpdump on the BIG-IP, you should be able to see the HTTP response for the failed EWS requests.
We recommend using the v1.2.0 iApp version in any case, since it has many fixes and added features over the version you are using.