Patients receive leucovorin calcium IV over 2 hours and fluorouracil IV over 30 minutes beginning 1 hour into leucovorin calcium infusion on days 1, 8, 15, 22, 29, and 36. Patients receive oral triacetyluridine every 8 hours on days 1-3, 8-10, 15-17, 22-24, 29-31, and 36-38 beginning 8 hours after completion of each fluorouracil infusion. Courses repeat every 8 weeks in the absence of disease progression or unacceptable toxicity.