The past couple of times I've started my car today, it starts up normal, revs to about 1800 rpm, which is normal, but then the rpms drop right off and my car stalls. It did it two times in a row. If I start it and rev it for about 5 seconds right when it cranks, then it wont stall. But if I dont rev it, it starts and then stalls right after. It only does it sometimes, not all the time. What could it be?