Ethernet downloading speed stuck at 100 mbps

My Ethernet plan is 250mbps as I take a connection a year before, but from yesterday I check my internet speed in my desktop in ookla speed test its showing only 100 mbps. but before its show 250 mbps and then i check the speed in all my other devices, there it is showing correct (250 mbps), but in my desktop it is not showing, so i decided to change the ethernet cable to (cat 6) but nothing change and after that uninstall the drives and reinstall for sometime it shows 250 mbps after that again it comes to 100 mbps and i also change the Speed & Duplex setting to 100mbps to 1.0 Gbps but it doesn't changes its only stuck at 100 mbps and sometimes when i plug out the ethernet cable and insert again then also for sometimes it shows 250 mbps speed but again it automatically switch to 100 mbps.

Please help me out with a permanent solution.