pc keeps dropping in internet speed

[Cosimo43]

my pc has a ethernet cable to the router but when i use a speedtest it starts around 300mbps and drops to 30 / 60 mbps. but when i connect my cable to my laptop it has a contstant 500 mbps. and when i download files it begins with 10mb/s but slowly goes to 200kb/s and then it drops to 0kb/s and after sometime it goes up to 200kb/s and it repeats.

[01davie]

Are you able to test from a single source, In the past I have single site downloads to test grabbing a file so I can compare the performance between a cable and wifi. 

[Cosimo43]

i have only ethernet