Have you quit your apps lately?
Double tap your home button and hold an app, then click the minus sign for every app. If this works for you, do this whenever your iPod touch lags or is slow to respond. (This is only for iPod's with iOS 5 or later I think.) This will also save battery too.
Hope this helps,
Contact the developer/go to their support site if only one app.
Restore from backup. See:
Restore to factory settings/new iPod