Fps Drop

When i join a match it starts at pretty high 200-300fps then out of no where drops not to like 15 at once but it will go 180, 150, 100, 75, 25, 15, 0.6 and it will stay around the 20s many people i know have experienced this problem and i know nothing is wrong with my computer because i had it checked by specialists and got my modem and internet checked so anyone have a solution?