03 RR shuts off while driving

So I've had this truck for a couple years with a few mechanical headaches. Today my husband was driving it to work in stop and go NYC traffic when it shuts off completely on the highway. Everything was working (i.e. air conditioning, radio etc) it just wouldn't move. After a couple minutes he was able to restart the truck and head back home to switch cars but it shut off again when he got off the highway. Each time it shuts off nothing else seems amiss but the check oil light does come on. After he got home he checked the oil and it seemed ok (I had an oil change about 2900 miles ago). He did mention that prior to it shutting off he noticed as he decreased his speed it seemed to start having issues but at higher speeds it was fine. Any ideas?