Dan:

The vapor recovery canister under the hood is your culprit.  It sits
above your left front tire. It is a black plastic box with a vent or
two in the top.  Put your nose near the vents when you notice the
smell.  Likely it has a cracked hose or is overwhelmed by the gas vapor
from the fuel tank.  It is also possible that the solenoid that feeds
the fumes into the intake manifold is faulty and not letting the
recovered vapor get vacuumed out and burned in the engine.
HTH
-steve