I had the same problem with my VPN dropping the connection after a few minutes. I tried a lot of things. What worked for me was realizing that OpenVPN is a part of most VPN software.

OpenVPN requires that the VPN user to have administrative privileges (https://doc.pfsense.org/index.php/Windows_OpenVPN_client_cannot_reach/route_over_VPN).

Windows 10 does not run in administrative mode automatically without specifically telling it "run as administrator". Not all programs used in Windows 10 need administrative rights to run properly. However, OpenVPN in Windows 10 does need administrative privileges to run properly.

To assign your VPN program administrative privileges in Windows 10, "right-click" on the VPN icon on your icon page. Then click on "properties". Then click on the "advanced" tab. Afterward, check the box that says "run as administrator". Next click "apply". Last, click "ok". That should do it.

You may have to assign yourself administrative privileges before doing the above (click "windows" key and "x" key at the same time, then click "run as administrator"), if the "run as administrator" box does not appear in the "properties"-"advanced" tab.

If you want to run your VPN as a non-administrator, you can (http://www.hacker10.com/computer-security/use-a-vpn-on-a-computer-without-admin-rights/).

Apparently, OpenVPN routes traffic while you are using your VPN. If OpenVPN cannot route the traffic, it is an error. If an error occurs, the VPN connection can drop out.

Again, OpenVPN is a part of most VPN software so run your VPN with administrative privileges or non-administrative privileges and be "happy" again!

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This may sounds obvious but have you forwarded the correct ports through sonicwall for your VPN?
If you have, do you have any QoS running, it could be worth checking that VPN has priority over VoIP etc.
Do you get disconnects all the time or is it only during busy periods? You may be overloading your server, perhaps it's time for more RAM.