the car will randomly overheat sometimes driving down the highway, stop-and-go city traffic or idleing.
it happens maybe 1 in 10 trips, but the other 9 could be for 100 miles with no problems.
it has a very slow leak, i havent noticed any water actually coming out but i notice a drop in the resorvoir from time to time, i dont know if this could be from the car getting so hot it evaporates or not.

i have replaced the thermostat, and i think they had a pressure test done on the radiator and a compression check on the cylinders, however im not sure about those.
it does not produce any smoke out of the exhaust but people have suggested it could be a cracked head.

any ideas?