Hmmmm 5 miles to overheat, which coincidently is about the time the thermostat takes to open. I know you say stat is good, try starting from cold and leave running and see if a, heater is working which indicates water pump working and then b, keep your hand on the top hose as the thermostat opens it should become hot very quickly, then as the cold water exchanges back into the engine the stat will close for a short while. Check for the hoses getting hard indicating faulty rad cap (unlikely) of head gasket (more likely). But do all the cheap checks first. Take of the grill and feel if the rad is warm all over with no cold spots, the water comng out of the rad should be significantly cooler than the hot going in at the top hose.
regards
Dave