1997 Schwinn Moab2 cross-country racing, highly modified, rebuilt many, many times. very fast!
you might have worn out gears on your cassette. to inspect them, look at them and see if they're shaped like an ocean wave instead of a half circle. the chain wears down the tips of the gear lobes and your chain will slip over the teeth when you pedal extremely hard. The chainrings on the crank will wear out eventually too.
Also check your chain wear... in fact, a chain should wear out faster than the gears it came with, so it's a more likely culprit unless you've already changed your chain once or twice. As usual, Sheldon can tell you the drill: http://www.sheldonbrown.com/chains.html#wear