I just moved to Co where I now get to do some more "MT" biking. I have a pair of formula hydraulic disc brakes (cross country and please dont tell me to get downhill brakes). My problem is on the downhill portion of my rides, the brakes heat up so much that it builds a lot of pressure in the brakes, at some points causing the brakes to almost stay locked.
when you notice the brakes overheating you need to stop and let the brakes cool down. You might be using a rotor that is too small for the riding you do. A larger rotor would relieve some of these problems. An open system is less susceptible to overheating. A system such as the Hope Minis comes highly recommended for XC.
It sounds like you are using a first-generation design caliper. The original closed system units were often prone to this effect. Hence open systems and different fluids. You may want to look into using a higher-temp brake fluid. Some automotive or motorcycle types are compatible and perform better. I have been running Shimano Deore Hydraulics for 2 race seasons on 2 bikes and they have hardly gotten warm calipers.