As long as the Center is overloaded, the Gateways are expected to drop.
you need to focus on the Center first.
The sizing tool is an estimation, there are cases where it will suggest something too low.
If it is sized correctly, I suggest to open a ticket with support to they can inspect the Center logs & BLG files to see if indeed it is overloaded and if it is because of lack of resources, or if there is something specific that is jamming the pipe...