I made japchae last night for 10 people for my wife's birthday dinner. It was fantastic! I've made it before from various recipes and it came out OK. But last night was the best ever. I think there are a few things in your recipe that make the difference: 1) seasoning the noodles right after cooking with soy sauce and sesame oil; 2) adding garlic to the cooking beef (instead of just marinading the beef); and 3) finishing the dish with the sauce of soy sauce, sesame oil, sugar, and black pepper. (although I did cut back on the sugar a little).
Thanks so much. This is one of my wife's favorite dishes and now I can make it for her any time.