Thanks for you input guys.

To reply on your thoughts:
- No, I don't use anything on my lips.
- This has happened in both mic'ed settings and none mic'ed settings.
- No, I was not anxious at all (Oh but I've been there a few times in the past. Believe me..

)
But when I look at what I wrote in my first post I see that I give the impression of barely playing at all during the soundcheck. This is not quite correct. We played 4 or 5 tunes or so where I try to be as careful as possible. I've been thinking about this quite a bit the last few days and in retrospect I might have gotten a bit carried away on "Digging on James Brown" - you know, the Tower of Power song, during the soundcheck. I apologise for being misleading.
I still think it's a bit odd since I never have this problem when there's no soundcheck before 4 hour gigs. My lips can take 15 minutes off but not 3-4 hours?!?
You might have a point on the warmup Tony. This last time I didn't get to warm up for as long as I would like. Still, previously I have been able to warm up good enough and the problem occured..
Being an amateur I guess I have to live with it. I can always skip the soundcheck allthough I don't like to do that..