Most comon issues are, things like, packet loss, wrong MCU size high jitter, to high bandwidth usage on a slow link, ...
Could you check on the MCU, endpopoints, call control (vcs, ..) how the call/media statistics look like?
Any network statistics availalbe? Always good to look into.
Which MCU software do you run? (I would run the latest).
Also check on the fw versions of the other involved components.
Which media-codecs, which bandwith is used?
Does it happen with other locations or point to point calls as well?
If its multiple users seeing the same behavior coming from a specific endpoint like that C90 I would start
checking the path in between the C90 and the MCU.