Need a bit more info on the symptoms, year of car. How hot is hot. Overheating or normal running temp? will it idle (tick over) when hot? If it coughs and splutters when at normal temp it probably is an air flow sensor/valve or an Idle Air Control Valve (if fitted). If it's an old one (carburettor) it'll probably be a choke fault. If your concerned that the garage is taking you for a mug then i'd suggest a second or even a third opinion.