I have had severe cramps while riding hard trails also, but it is usually my own dumb fault. I may have indulged in some beverages the night prior and did not drink enough water. Being dehydrated makes me cramp up on the tough trails. I cramped up in my first MTB race last weekend, cost me several minutes…but still placed 7th. STAY HYDRATED!
You should try the Eibsee trail. It has been a while since I have gone, but I do plan on going again this coming spring/summer. A buddy and I went on this trail around the lake and it was a great workout and had awesome scenery. I have a bunch of pics. Anyways, you have to pedal from garmisch through Grainau, and when you see the lift for the Zugspitz on your left, you should see a gravel trail on the right directly after the Zugspitze lift. Take that trail and follow the signs for Eibsee. I don’t know the exact mileage of the trail, but it is quite a haul. You will spend a couple or few hours going up. Then you have some nice downhills around the lake. Like I said, I plan on going this next spring or summer, so I can get proper directions in mile increments, with descents, since I have a nice computer now.