Patients receive irinotecan hydrochloride IV over 90 minutes followed 5 hours later by leucovorin calcium IV over 2 hours and alvocidib IV over 1 hour immediately followed by 5-FU IV continuously over 48 hours beginning on day 1 of weeks 1, 3, and 5. Courses repeat every 6 weeks in the absence of disease progression or unacceptable toxicity. Cohorts of 3-6 patients receive escalating doses of alvocidib and 5-FU until the maximum tolerated dose (MTD) is determined. The MTD is defined as the dose preceding that at which 2 of 3 or 2 of 6 patients experience dose-limiting toxicity. Ten additional patients are treated at the MTD.