The transmission doesn't effect idle speed, at idle power is not transmitted to the transmission. Power is only transmitted when the torque converter gets up to a certain rotational speed. It will not effect the idle speed. Your problem is engine or engine control related and needs to be checked out. The car should have a catalytic converter but if it not there it is not causing a restriction in the exhaust system that would prevent the exhaust gasses from exiting the engine, which could cause stalling or inability to idle (in severe cases) or accelerate.