the car i am buying dealer says car stalls some times when car slows down and is in neutral he says it revs varies from 500 rpm to 1000 rpm any idea how to fix this he says he thinks it is running rich on gas cause of all the mods and all it needs is a new tune any ideas how to fix and where to get fixed how much thanks

it is a 03 cobra with several upgrades cams, larger injectors, headers, etc