Internet connection is lately been dropped till 100 Mbit/s

[Jaazu]

About 4 days ago my 500Mb Ziggo connection droped to between 90-100Mb max and nothing I do seems to get the speed back.  My ziggo provided modem is in bridge mode, the router behind it hasn’t been changned in  6+ months (rebooted a few times the last few days trying to resolve this).  I’ve done speed tests at the second position router hardwired and still 90Mb.  I’ve tried on mutiple hard wired computers on my internal network.  I’ve copied files between on network machines at full 1gb.  The lead router is a Firmware Version:3.0.0.4.382_51641  RT-N66U from Asus.  It’s also been rebooted multiple times since this started.  

Any help at all would be appreciated. Feel free to ask technical questions as I do networking part time for my profession. (speeds have been checked with speedtest.net and with dslreports for verification)

[DennyW]

Have you already checked the ethernet cable between the modem and the router?

[Jan F.]

A dropping connection to 100 Mbps or less is typical for a faulty cable. Replacing the cable bij a new cat 5E or better will solve the problem.

[Jaazu · Accepted solution]

RESOLVED:  QOS had become enabled on the lead router during a restart that updated it’s firmware.  Disabling QoS returned the internal connection to it’s full speed.

[DennyW]

@Jaazu 

Thanks for the update.
So, although you didn't make any changes yourself, the automatic update function in your router did. (Restored default values?)

[Cecilia]

I'm happy to read that the issue with your internet connection is been resolved and that you have your full speed back @Jaazu  Thankyou for your update!