Uh-huh. So when the ISP slows down your response times slow down do you mean you're still playing as fast as usual but your moves aren't getting input as quickly as at other times? It's very possible your ISP has one or more "rush hours" a day when traffic is higher than usual so everyone is getting slowed down.
Next time try describing your OS, browser and ISP. What's your download/upload speeds? Maybe you'd see an improvement if you upgraded to a faster connection. SuddenLink my ISP allows me to upgrade/downgrade anytime I want with pro-rated fees for the time I'm on different plans (instead of making me pay for a months service under one plan). If your ISP does likewise try using a faster connection.
http://blog.chess.com/NimzoRoy/diagnosing-and-fixing-internet-problems
I have noticed that during the time of day my pc seems slower as does the response time of my moves. I played three blitz tournements yesterday and the first one was terrible as I played during a time many in my time zone were online. I went 1 and 7. Later that evening I played again and the second time I was 7 and 2 and the competition every bit as good if not better. The third I went 8 and 1 (first). My opening and move time are the same. Does anyone else notice this or is it unique to my internet provider? I am getting old but this does not seem to effect my decision making or move time. Thanks to all I amagrandmother3.