If coolant is overflowing, there's too much pressure in the system. This is usually caused, in my experience :), by two things - overheating, or cracked head gasket.
Actually, 4 things - it could simply be a stuffed radiator cap, or thermostat. About $30-$50 to replace those two items; they probably need doing anyway
And since it's not reading hot, I suggest it's not overheating.
Proud TQE Operator and President of SEFAG - the Society for the Elimination of Ford Automatic Gearboxes
Pacemaker headers + 2.5" exhaust
EF Fairmont interior
Lowered King springs
5 speed manual
Newest mod: EF Ghia wiring loom and dash