You could try a different thermostat as one of the others recommended. the Stant brand thermostats (chrome with a spring on the back) you see at kragen, autozone, pep boys etc. have a pretty small opening that could be a problem if that is what you are using. If you can find a Robertshaw, they have a much larger opening, and are the same ones repackaged by Mr. Gasket and probably milodon too. The opening on the robertshaw is almost 2x the size of the Stant style thermostat and I'm pretty sure the robertshaw is original equipment from mopar. They are copper/brass construction with no exposed spring on the back side. Also, I think you might be on the right track with the shroud too. lots of cars with electric fans have flapper doors in the shroud so they don't block too much airflow on the freeway, your shroud definitely covers a good part of the radiator core....hope you get it sorted out.
