Very slow after 10mb to 20mb upgrade

[bluesuit]

Recently upgraded from 10mb to 20mb.

However I find it very hard to get anywhere near 20mb.

If I re-boot the modem and router I do get a brief and reasonable speed:

https://www.cableforum.co.uk/images/local/2009/11/42.png (http://www.speedtest.net)

The problem is that this will last for around 10 minutes or so and then I'm right back down to this:

https://www.cableforum.co.uk/images/local/2009/11/43.png (http://www.speedtest.net)

The speed will be this slow until I re-boot again and then I get the high speed for another 10 mins. Very odd.

My modem details are:

Type : Ambit ETH/USB Combo Cable Modem
Cable modem : Ambit Cable Modem
Downstream Receive Power Level : 5.56 dBmv
Downstream SNR : 33.79 dB
Upstream Transmit Power Level : 49.00 dBmv
Network Access : Enabled
Maximum Downstream Data Rate : 20480000 bps
Maximum Upstream Data Rate : 768000 bps
Maximum Number of CPEs : 1
Software Upgrade File Name : ntlhm100_ntl0003.cpr
Software Upgrade AdminStatus : allowProvisioningUpgrade
Software Upgrade OperStatus : completeFromProvisioning
Downstream Lock : Locked
Downstream Frequency : 331000000 Hz
Downstream Modulation : 256 QAM
Downstream Receive Power Level : 5.59 dBmv
Downstream SNR : 34.13 dB

Any ideas why this would be happening?

[Ignitionnet]

According to your config file you have an ntl home 100 modem - this isn't acceptable for 20Mbit.

Contact virgin and get a newer model.

[bluesuit]

Thanks for the reply. Yes, contacted VM and they confirm that modem is dated. Are sending a new one out.