DISEASE CHARACTERISTICS: Histologically confirmed advanced solid tumor for which no standard therapy exists or which has progressed or recurred after prior therapy OR Lymphoma for which no further therapeutic options exist No hematological malignancies (e.g., leukemia) No known brain or leptomeningeal disease unless previously treated with radiotherapy, not currently treated with corticosteroid therapy, and no clinical symptoms present

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: ECOG 0-2 Life expectancy: At least 12 weeks Hematopoietic: Absolute neutrophil count at least 1,000/mm3 Hemoglobin at least 9 g/dL Platelet count at least 100,000/mm3 Hepatic: Bilirubin no greater than 2 mg/dL AST or ALT no greater than 3 times upper limit of normal (ULN) (5 times ULN if due to tumor involvement) Alkaline phosphatase less than 2.5 times ULN Renal: Creatinine no greater than 2 mg/dL OR Creatinine clearance at least 60 mL/min Cardiovascular: No unstable atrial or ventricular arrhythmias that require control by medication No ischemic event within the past 6 months Other: Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception Abnormalities that would affect absorption of study drug allowed at investigator's discretion No other severe disease or psychiatric disorder that would preclude study No phenylketonuria No known hypersensitivity reaction to artificial sweetener aspartame (e.g., Nutrasweet)

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: Recovered from prior chemotherapy for cancer No other concurrent cytotoxic therapy Endocrine therapy: See Disease Characteristics Radiotherapy: See Disease Characteristics Recovered from prior radiotherapy for cancer Surgery: Recovered from prior surgery for cancer Prior gastrointestinal surgery allowed