Seems to me like they are approaching the problem in a responsible way. They are trying to diagnose the problem which is the right thing to do. They could have just started replacing parts which would have cost you a lot of money and been more profitable for them. Replacing the radiator cap was an easy/cheap first step. From here, a non contact thermometer can be used on the radiator to check for cool spots indicating blockages. Also, verify your radiator fan is coming on when the engine heats up....they may have already done this. Finally, If you have not replaced thermostat in the last 3 years, do that.