Chronic Pancreatitis: This condition is suggested by the patient's chronic abdominal pain, episodes of loose stools, and elevated serum amylase levels. Chronic pancreatitis can lead to malabsorption, causing loose stools, and the elevated amylase supports pancreatic involvement. The fact that lipase is normal does not rule out pancreatitis, as amylase and lipase can be elevated at different times during the course of the disease.

Gastroesophageal Reflux Disease (GERD) or Peptic Ulcer Disease: These conditions can cause chronic abdominal pain and vomiting. Although they do not directly explain the elevated amylase, they are common and could coexist with other conditions.
Small Intestine Bacterial Overgrowth (SIBO): This condition can cause abdominal pain, vomiting, and loose stools due to bacterial overgrowth in the small intestine. It might not directly explain the elevated amylase but could contribute to the gastrointestinal symptoms.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause chronic abdominal pain, diarrhea, and vomiting. While they might not directly cause elevated amylase, they are important to consider in the differential diagnosis.

Pancreatic Cancer: Although less likely given the chronic nature of symptoms and the absence of significant weight loss or a palpable mass, pancreatic cancer must be considered, especially in an elderly patient. It can cause obstructive jaundice, abdominal pain, and elevated pancreatic enzymes.
Intestinal Obstruction: This is a surgical emergency that can cause severe abdominal pain, vomiting, and obstipation (inability to pass stools or gas). It might not directly explain the elevated amylase but is crucial to rule out due to its urgency.
Mesenteric Ischemia: This condition involves insufficient blood flow to the intestines and can cause severe abdominal pain out of proportion to physical findings, vomiting, and possibly diarrhea. It's a medical emergency and must be considered, even though it doesn't directly explain the elevated amylase.

Zollinger-Ellison Syndrome: A rare condition characterized by excessive gastric acid production due to a gastrin-secreting tumor, leading to peptic ulcers, diarrhea, and abdominal pain. It could potentially cause elevated amylase if there's associated pancreatitis.
Whipple's Disease: A rare, systemic bacterial infection that can cause malabsorption, weight loss, abdominal pain, and diarrhea. It's very uncommon but should be considered in cases of unexplained, chronic gastrointestinal symptoms.
Eosinophilic Gastroenteritis: A rare condition characterized by eosinophilic infiltration of the gastrointestinal tract, leading to abdominal pain, vomiting, and diarrhea. It's an unlikely cause but could be considered if other diagnoses are ruled out.