Three main possibilities: the radiator cap is leaking under pressure, or
there's a pin hole in one of the hoses, or there's a hole in the radiator.

You don't mention rough running or white exhaust smoke, so a failing head
gasket is unlikely.

You also don't mention any coolant smell in the cabin, so the heater core is
not leaking.

When cold, pull the rad cap off and look at the coolant. Is there an oily
film or grey-white goo? If so there's oil in the water, and the head gasket
has failed.

Does the rad cap rubber seal look damaged or cracked? If so, replace it.

Squeeze all the hoses: they should be firm, not hard or soft and squishy. If
not firm, have them replaced. Look at the 1" hoses that connect the engine
to the heater, through the firewall. There's also some small hoses that
connect part of the heater system to the main coolant hoses (rear of the
engine). If these are contaminated with oil, or soft and squishy, replace
them.

If the engine side of the rad is corroding away, its time for a new
radiator, with all new hoses.

Lastly, look at the overflow bottle. There's a gasket under the lid. Is this
sealing correctly? This is not essential, as the main sealing is done by the
radiator cap.

Stewart DIBBS
www.pixcl.com/lancerproject.htm