Speed Test

I recently upgraded from 50mbps to 100mbps at the beginning I was receiving around 94. Then it dropped off and now I am doing good to get 34mbps. Cox has been out trying to figure out why. Cox always uses speedtest.net I was told today that it is a 3rd party and not reliable! So I went and tried 3 different ones all coming up with the same results. So does anyone know a way to check that is accurate? In case any want to know we have (cox) literally ran a fresh line from tap to modem to laptap with Ethernet cable same results 33mbps! When tech called in they told him results in office show we are getting 100mbps. I am suppose to take their word on it! When watching a movie I getting the spinning circle for movie to download. I didn't have issue when it was 50mbps.