Boxsters had cable operated throttles until 2000MY when Porsche went to E-gas that opens the throttle electronically even though the gas pedal is connected by a short cable to the sensor that sends input to the ECM. Sounds like the OP's 99 may have a misrouted cable which should be a easy enough fix but makes you wonder if a major repair was being done & the cable was misrouted then. Could be a good thing (New Engine ) or amatuer maintenence.
