I found a Korean cookbook in B&N, and got it because it has what look like good recipes, and also some nice pictures and commentary - The Food Of Korea - Authentic recipes from the land of the morning calm. Here is the Japchae recipe from that book -
Beef -
6 1/6 oz beef filet or sirloin, shredded
1 TBSP light soy sauce
1 TBSP sugar
1 tsp sesame oil
1 tsp finely chopped garlic
4-6 scallions, cut in 1 1/2" lengths
1 TBSP vegetable oil

Vegetables-
2 TBSP vegetable oil
2 onions, thinly sliced
4 dried black mushrooms, soaked to soften, stems discarded, caps shredded
4 wood-ear mushrooms, soaked to soften, then thinly sliced
1 med carrot, finely julienned
1/2 green bell pepper, finely julienned
1/2 red bell pepper, finely julienned
1 TBSP light soy sauce
1/2 cup water
1/4 tsp salt

Noodles-
1 TBSP oil
13 oz potato starch or cellophane noodles, soaked in warm water 10 minutes
2 TBSP sugar
1 TBSP sesame oil
1 TBSP soy sauce
1 tsp salt
1/4 tsp white pepper

To prepare beef, put meat in a bowl and add soy sauce, sugar, sesame oil, and garlic. Massage for about 1 mn, leave to marinate for 30 mn. Heat oil in wok and stir fry over high heat until done, transfer to bowl (they don't mention scallions, I would put them in to marinate to)
To prepare vegetables, heat oil in wok, and add onions. Stir fry over med heat until transparent, add mushrooms and stir fry 2 mn. Increase heat slightly and add carrot and all bell pepper. Stir fry 2 mn, then sprinkle with soy sauce and salt. Add water and stir fry until vegetables are soft and liquid has dried up. Transfer to bowl with beef.
To prepare noodles, heat oil in wok, add noodles, stirring for a few seconds to coat well with oil. Add sugar, sesame, soy, salt and pepper, then add the reserved beef and vegetables, stirring to mix all ingredients thoroughly. Serve.