Do you mean the pc is going to sleep, or it's just turning the monitor off? The difference is that a sleeping pc is normally a lot quieter and normally flashes its power light. A pc with the monitor off is just as noisy as normal and still has the power light on, and may still flicker the hard-disk light occasionally. If the pc is actually sleeping, check your bios for any settings relating to sleep/suspend. You want mode 3 sleep/suspend if possible, and there might be a setting for re-running the graphics card's bios/init when coming out of sleep, you can see if that helps.
When the system is lagged, open task manager (ctrl+shift+escape) and see which process is using the most cpu time (ignoring the "system idle process", obviously). Also check that you're not using close to all of your physical memory. If you are, you're probably experiencing the effect of swapping ram to the page file, and you need to either kill things using a lot of ram, of get more ram installed. If that doesn't identify an obvious culprit, open the resource monitor from the performance page of the task manager, and look for processes doing a large amount of disk reads/writes. Again this can cause serious performance problems, and you should look into whether you really need the offending process.