In reply to Cake:
I get cramp in my hands/fingers when I am very tired. I remember somewhere high on
Fiesta de los Biceps, in Riglos, seeing the middle finger of my right hand literally curl in on itself, completely against my will, as the forearm muscle cramped. I spent the rest of the pitch (and route) trying to climb one-handed, and in considerable pain.
I have had it come and go over the years - it usually occurs when I have been climbing down the wall a lot (i.e. short bursts, 2/3 hours max) and then go for a day's cragging (i.e. effort spread over 8ish hours), so I get tired and cramp.
Unfortunately, I have no specific advice to help get rid of it, beyond "climb outside more", which is probably the best advice 99% of the time anyway! I hope you manage to find a way around it.