Ram Speed issue

I have a dell inspiron 15 3515 laptop and when I got the laptop the Ram was running at 3200 Mhz and I recently updated my BIOS as I was starting to see issues with playback so I updated all the drivers and the BIOS but the issue still remains, video streaming sites lag and sometimes I get a 'Restart your device if the issue persists' and even the Ram speed has decreased to 2400Mhz , I have not changed the ram but after the BIOS update which was said to be 'critical' I now have my ram running at 2400 Mhz and there is no way to change it in the BIOS