Windows Update Server CRC Verification errors if downloaded through Fortigate

I have been troubleshooting a problem since August on my Windows Update Server where certain files would have CRC Verification errors. Finally today I routed my server physically around the Fortigate and everything downloaded fine. I am not sure where to start troubleshooting this. I don't seem to have other problems with downloading files for my users. It seems to be specific to the type of traffic this server is doing. Any suggestions?