Car dies periodically

I'll start my car up, drive somewhere and it'll
be running fine. I'll stop, turn the car off to
go somewhere, come back out, start it up to
drive somewhere else and the car will die.
Turn it off, put it in park, turn it back on, put
it back in drive and it'll die again. When it
finally does get going the check engine light
comes on and I'll give it some gas, it'll drive,
but then it'll stall and get out of the stall it's
self. I'll let it sit over night and the check
engine light goes off. Today I took it to the
dealership with the check engine light on
and the codes showed a problem with the
transmission , bit they couldn't find
anything wrong with it. This has happened
more than a few times. Once it smelled like
sulfer when it did this and the exhaust
smells like fuel. It seems like it acts like this
when there's less than a half a tank of gas.
Any suggestions would be helpful. The car
has 75,946 miles on it and had a massive
vacuum leak in it that was repaired and had
a tune up with 72,000 miles on it.