1985 chevy, reman motor with 150+K on it. in the past three weeks I have been having a slight overheating problem when stopped. After short trips, the overflow tank will boil over, when cool I can tell the radiator is emptying into the overflow tank. No white smoke from the pipes, no noticable performance loss, just the missing coolant, no leaks in the radiator, hoses, water pump, etc....just changed to a 185 thermostat, still no help. Today it got worse, coolant boil over from radiator but never returns, at idle getting actual coolant out of the exhaust(the truck is parked). A buddy suggested an intake gasket leak, hence smoke off the line, fine at speed.(vacume) another said maybe a head gasket with a leak between exhaust and water jacket hence the bubbles opening the radiator cap and allowing coolant out but not back in. The water in the radiator drops when I rev it so I know the water pump is good. Anyone with similar problems and remedies???