Big speed drop?

[#1]

I noticed my internet was really very sluggish so went to mybroadbandspeed for a quick test and it was running at around the 300 mark, I switched to my normal account and the speed jumped up to 10,000.  I did notice this once before happening but figured it was a one off, is there any particular reason the IPV6 account would drop by such a significant amount?

[#2]

Maybe the disconnect/reconnect of the PPP session was enough (connected to a different gateway), have you tried again with the IPv6 login?
How do your v4 & v6 speeds compare here: http://www.thinkbroadband.com/ipv6/speed-test.html (with the IPv6 login)?

[#3]

I rebooted the router first to make sure it wasn't a router problem before changing over login details.  I'm downloading at the moment but will switch back to IPV6 after a speedtest to see if the speed is still slow.
/edit --
IPV4 speed test on normal account

IPV6 account - no image...
IPV4 Down = 7.21Mb | IPV6 Down = 10.17 (expected, my IPV4 speeds on the IPV6 account have always been 7.5 tops compared to my normal speeds)
So the speed issue has fixed itself for now, but if it happens again that TB speedtester looks pretty useful!  Doesn't explain why there was such a large and sudden drop mind you.