My problem, which was identical to jdailey's, was fixed by replacing the Fuel Pressure Regulator on the Fuel Rail spider. I did all the work myself in about an hour and a half and the part can be found on E-Bay or at AAAParts.com for around $50.00. Here's a great article describing the problem:
http://www.samscars.com/mage7.html. My Jimmy now runs perfectly!