2006 Honda Civic overheats after 40 miles

I have an 06 Civic EX. In Feb. it overheated after driving it about 40 miles. I pulled off the highway at my exit and it overheated. Drove it home with heat cranked to keep it below red zone. Had thermostat replaced and coolant flushed. 2 1/2 weeks later, overheating again, after 40 mile trip! This time temp sensor was replaced (and it needed coolant). 2 1/2 weeks later, overheated again, needed coolant. Diagnosed by local shop as Head Gasket! Contacted Honda, took it to dealer, NO PROBLEM with car, could not make it overheat (coolant had already been replaced). 2 1/2 weeks later, 40 miles into drive, overheating again. I believe it is head gasket, but Honda dealer said compression was fine. Ideas???