Insert meat thermometer into thickest part of roast, making sure it does not touch bone or fat.
Saute onion and garlic in butter in a medium saucepan until tender.
Stir in remaining ingredients; bring to a boil.
Reduce heat, and simmer, uncovered, 5 minutes, stirring occasionally.
Remove sauce from heat.
Set roast and sauce aside.
Prepare fire in a covered grill.
Place roast on grill.
Cover with lid, and open vent.
Cook over low coals 3 hours, turning roast occasionally.
Baste roast with sauce.
Cover and cook an additional hour or until thermometer registers 160° (medium).
Baste frequently with remaining sauce.
Let stand 10 to 15 minutes before slicing.