Unstable Internet with MTU not default Firmware 1.30 700H

Hello, I would like to tell you that I changed the MTU to 1500, the default, thinking that was the problem, but it did not solve the problem. The internet is very unstable, there are micro outages, the problem is that we handle billing by sending XML to our certifier, all these small 1K files, the Firewall stays stuck or on hold and does not work correctly, all this happens with version 1.30, I go back to 1.21 and it works without problem. The DNS searches sometimes respond, sometimes they don't, the internet speed tests work perfectly when downloading but when uploading it either loses the signal or gives a speed lower than 500k when we have 200Mbps. Please check this version to see if there is no problem with the scanning of small files.