Okay, One free test. Since your connection is IMAP. Download Thunderbird. (free email client) and connect to the same account via imap as well. Does Tbird have the same issue... if so, then you can rule out outlook I might even make a test account on your domain, and use the smarter-mail import feature to populate it with the same folders and test that, You could have a very rare corruption of your GRP file in SM. That is rare and unlikely, but possible.
Run a constant ping to the mail server while syncing, do you have any time outs?
Trace-route: run a trace or as many as you can, to identify the location of time outs while you are performing the sync..
Disable your machines A/V software, does that clear it up?
And finally to the logs. When you connect via IMAP you will see something like this.
then the sync, and the number of messages,
then you will see the server serving the messages,
They are a bit overwhelming at first, and you should learn to read them. SM has great logs. I am grateful to them for that. They are detailed and fairly easy to understand. Email can be daunting, but you can get through this.
Remember kids, every time a spam message gets blocked, a nerd gets their glasses. spamhurts/July 15