Are there any users that are not seeing the problem?
You are assuming the problem is the RB5009.
Can you verify that the problem does not exist for a PC that is directly connected to the LAN that the ESXi host is connected to (in other words, if the problem exists without the RB5009 involved in any way, then trying to solve the problem by changing something on the RB5009 is going to be a waste of time.)
Intermittent problems are much harder to solve than problems can can be reproduced at will.
It's possible that the problem is in the RB5009, but as @johnson73 mentioned, it may not have anything to do with the RB5009.
Are there any uses of RDP on that are on the LAN that the ESXi host is on? Do those users also experience pauses? If so the problem is more likely to be in the ESXi.
Intermittent problems are some of the toughest to troubleshoot, so do anything you can to try to isolate/elimiate things as the cause. If you can remove the RB5009 from the test, and can verify that the problem goes away for users that aren't going through the RB5009, then at least you will know where the root cause has a low probability of being. But then you still have many other possible problems areas, network, router, even the PC that is the client.
Good luck narrowing down the issue. I think all the advice that @johnson gave were good things to investigate.
But you can also look at using something like wireshare to watch traffic (both on the client side and the ESXi side of the link) to look for clues (lost packets, etc.) But if you have never used wireshark, that's goint to be a whole new learning curve.
But if I were you, I would verify that the problem does not exist when the WAN/VPN/Router is out of the path. Otherwise you may be trying to fix a problem you think is related to the RB5009 when if may have absolutely nothing to do with the RB5009.
When troubleshooting intermittent problems, try with the simplest possible situation first, and try to stress it to hopefully trigger the issue.
