Router on its last legs, or line fault ?

[#1]

My sync rate is around 13000, my ping is allways 5 ms, and my speed usually sits around 11 Mbps. The last 2 days my speed has been crazy. Going from 7 Mbps up to 24 Mbps. Ping has gone from 5 ms up to 26 ms.
Doing a speed test the speed sits at 2 to 3 Mbps for around 7 seconds, shoots up to 70 + Mbps, then drops back to 20+. I have changed gateways, restarted my router, my exchange is green.

[#2]

Maybe it's speedtest.net?  How long has your router held it current sync rate?

[#3]

Have you put any new anti virus / security software on? I think kaspersky used to mess up such results.

[#4]

Thanks for the replies,
My connection has been up for weeks ReedRichards, apart from the odd Peer terminate.
Nice one dvorak, I have been using AVG free for years but after my last format, after I put AVG on it refused to download anything, for example skype,it would get to 98% then would say it couldn't download it, same as with the new intel ethernet drivers. So I put Kaspersky on which was free with my motherboard disk. I have put AVG free back on and the speed test is back to normal.

Thanks, again.

[#5]

If you haven't tried it you might quite like Microsoft Security Essentials? It's lightweight, fast and nag free, or at least I've found it to be so. Just a suggestion though.