Gemcitabine, Docetaxel, and Capecitabine in Treating Patients With Cancer of Unknown Primary Origin

DISEASE CHARACTERISTICS: * Histologically or cytologically confirmed malignancy for which no primary origin has been identified despite routine workup, including the following: * History and physical examination * Chemistry profile and other blood work, including tumor markers with follow up on any positive findings * CT scan or MRI of the chest, abdomen, and pelvis * Mammography (for female patients) * Prostate examination (for male patients) * Stool guaiac * Measurable disease * Previously irradiated lesions are not considered measurable disease unless there is documented clear tumor progression in these lesions after completion of radiotherapy * The following tumor types or presentations are excluded: * Resectable disease * Tumors consistent with germ cell primary, as indicated by any of the following: * Midline tumor * Elevated beta human chorionic gonadotropin * Elevated alpha-fetoprotein * i12p chromosomal alteration * Prostate primary with elevated prostate-specific antigen * Females with axillary nodes as the primary disease site * Tumors limited to the peritoneal cavity consistent with primary peritoneal carcinoma * Neuroendocrine tumors * Squamous cell carcinoma involving cervical or inguinal lymph nodes * No symptomatic brain metastases * Prior brain metastases allowed provided patient completed definitive treatment with brain irradiation with or without resection PATIENT CHARACTERISTICS: Age * 18 and over Performance status * ECOG 0-2 Life expectancy * More than 3 months Hematopoietic * Hemoglobin ≥ 9.0 g/dL * Granulocyte count \> 1,500/mm\^3 * Platelet count \> 100,000/mm\^3 Hepatic * Bilirubin normal * Meets 1 of the following criteria: * Alkaline phosphatase (AP) normal AND AST and ALT ≤ 5 times upper limit normal (ULN) * AP ≤ 2.5 times ULN AND AST and ALT ≤ 1.5 times ULN * AP ≤ 5 times ULN AND AST and ALT normal * Albumin ≥ 3.0 g/dL Renal * Creatinine ≤ 1.5 mg/dL Gastrointestinal * Able to take oral medication * Intestinal absorption intact * No uncontrolled diarrhea and/or daily emesis Other * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective barrier contraception during and for 3 months after completion of study treatment * No other malignancy within the past 5 years except curatively treated carcinoma in situ of the cervix or basal cell skin cancer * No severe medical or psychiatric illness that would preclude study treatment * No peripheral neuropathy \> grade 1 * No history of severe hypersensitivity reaction to docetaxel or other drugs formulated with polysorbate 80 PRIOR CONCURRENT THERAPY: Biologic therapy * Not specified Chemotherapy * No prior chemotherapy for this malignancy Endocrine therapy * Not specified Radiotherapy * See Disease Characteristics * Recovered from prior radiotherapy * Prior palliative radiotherapy to areas of bony metastases allowed provided there is measurable disease outside the radiotherapy port * At least 4 weeks since prior radiotherapy * No prior radiotherapy to ≥ 25% of the bone marrow Surgery * See Disease Characteristics Other * No concurrent antiviral therapy