Bortezomib, Fluorouracil, and Leucovorin in Treating Patients With Metastatic or Unresectable Stomach Cancer

DISEASE CHARACTERISTICS: * Diagnosis of gastric or gastroesophageal junction adenocarcinoma * Metastatic or unresectable disease * Progressive disease after receiving 1 prior chemotherapy regimen for metastatic disease comprising 1 of the following: * Fluorouracil * Cisplatin and irinotecan * Capecitabine * Taxanes * Measurable disease * No esophageal cancer * No brain metastases PATIENT CHARACTERISTICS: Age * Over 18 Performance status * ECOG 0-2 OR * Karnofsky 60-100% Life expectancy * At least 3 months Hematopoietic * Absolute neutrophil count ≥ 1,500/mm\^3 * Platelet count ≥ 100,000/mm\^3 Hepatic * Bilirubin normal * AST and ALT ≤ 2.5 times upper limit of normal Renal * Creatinine ≤ 1.5 mg/dL Cardiovascular * No symptomatic congestive heart failure * No unstable angina pectoris * No cardiac arrhythmia * No acute ischemia by EKG * No significant conduction abnormality by EKG, including either of the following: * Bifasicular block, defined as left anterior hemiblock in the presence of right bundle branch block * Second or third degree atrioventricular block * No history of cardiac or cerebrovascular disease due to hypotension and tachycardia Other * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception before, during, and for 6 months after study participation * No ongoing or active infection * No other uncontrolled illness * No peripheral neuropathy ≥ grade 2 within the past 2 weeks * No allergy to boron or mannitol PRIOR CONCURRENT THERAPY: Biologic therapy * More than 4 weeks since prior immunotherapy Chemotherapy * More than 4 weeks since prior chemotherapy (6 weeks for nitrosoureas) and recovered Endocrine therapy * Not specified Radiotherapy * More than 4 weeks since prior radiotherapy and recovered Surgery * More than 2 weeks since prior major surgery Other * No concurrent highly active anti-retroviral therapy for HIV-positive patients * No other concurrent investigational agents