vx I hate to suggest it but you may want to have them pressurize the cooling system overnight and look into cylinders and see if you blew a gasket, all it takes is a pin hole. I recently had this problem on another car, no taletell syptoms (smoke, oil in coolant, coolant in oil etc) but it would overheat now and then, I found moisture in #1 cylinder. I ended up getting a can of K&W block sealer, followed the directions exactly and cured the problem without the expense of a head gasket job. This stuff really works but you must follow directions to a tee.