Well if you keep smelling gas is it possible there is a tiny leak in the hose? That would cause a lack in vacuum which might lead to hesitating.
(Pure speculation here).
If it was your tranny slipping it would be broken by now
The tranny went in my truck, I felt it start slipping and by the time I was 4 blocks to the tranny shop it was pretty much dead.
As well, I think only the clutch can slip in a standard and that would be smoking everywhere if it was slipping.