The last few days I have netted a significantly lower PPD value. When we see this, I think it's one or more of these:
1. The work just changed. Nothing we can do about that.
2. There's a bad set of work units out there. I had that happen a few weeks ago where a number of WUs got thrown out. Then as suddenly as it happened, it was gone and everything started running fine again.
3. Sometimes I slide the client down so I can play a game (rarely) or render something (more frequently). I now make it a point not to slide it to the "off" position because I was doing that and then forgetting to turn it on again. Aaaaa! I forgot to slide it back up last night, so it ran all morning and all day in the "idle" setting. At least it got some work done, but it's definitely slower that way. Bah!
4. Sometimes a reboot helps clear the cobwebs and cockroaches out. On occasion, I also need to reset my network. Now I do all of this as an every-other-week maintenance task. Usually, after I get significant Microsoft patches, I'll install them, then will restart the workstation and also recycle my router and DSL modem to get a brand new connection.
5. A point to remember for Windows 8 users: If I recall correctly, shutting off the computer from Windows and then booting actually results in a "warm boot". If you want to really reset everything, just do a "restart" instead. That's what I usually do.