I have an old Peugot Canyon Express mountain bike, about 20 years old, not sure of the exact components. When riding up hills in a standing position, my chain sometimes "slips", in that it seems to miss a sprocket or something, which of course completly throws off my rhythm and balance. The chain doesn't come off or move to another gear. It also happens when starting to pedal from a stop in a high gear, but only when standing. It seems to be a function of the of torque applied. as it only happens when I am standing and pushing hard on the pedals.

Any ideas or suggestions?