Just changed the map sensor. It was running fine for a day but has started acting up again. What could be the problem?

Well Ashley you need to do some testing , check fuel pressure , check for spark at the plug wires . Something is cutting out when warmed up , ignition control module , a ignition coil , crankshaft position sensor ! Check the basics first when acting up , spark ,if there is spark spray some carburetor cleaner into the engine , if it tries to start then you have a fuel delivery problem ! A engine sensor could be shorting out bring the 5 volt ref. voltage circuit of the engine computer ! If your not sure of these thing's instead of just replacing parts an wasting money you mite want to take it to a ASE certified repair shop !