Place potatoes in a saucepan with enough cold water to cover. Bring to a boil, cover, and reduce heat to medium. Cook 10 minutes or until just tender. Drain and place in a bowl with the clam sauce. Set aside.
Cook fettuccine according to package directions, about 8 minutes. Drain in a colander and set aside. Add the oil and butter to the pot and return to medium heat. Sauté the garlic and parsley 1 minute. Pour in the potato and clam sauce mixture and bring to a simmer. Add the cooked fettuccine and lemon juice to the sauce and heat about 30 seconds.