Me and my band always improvise solos, when practicing or performing, and I know what you mean. Some days you're on, some days you're off. It seems to me that I play better late at night after I've drank like a case of cokes... I get that feeling that caffeine is running through my veins instead of blood, and I'm totally pumped. Also, when I have a really ****ty day or a really good day I play better. On just an average day, I play normal, but when I've had a healthy dose of daily emotions, my playing is a lot more emotional, too. Also, when I'm playing for a crowd I tend to play less technically, but more emotionally. When it's just me and the band, I play a helluva lot more arps and blazing scalar runs... I guess I'm kinda scared of screwing up in front of a crowd ;)