[767Jockey]

I think David and Mr0077 have it right. The problem isn't that you added a shroud, it's the size and shape of the shroud that you added that is causing you the problem. Long story short, from the looks of the picture you posted, the rear surface of the shroud is much too close to the back of the radiator, effectively blocking all airflow through the radiator except for the area ahead of the hole for the fan. Effectively, you now have a radiator that is 16" in diameter, the rest of it has blocked airflow.

It was explained to me once that a shroud is in effect a huge sealed plenum behind the radiator. The puller fan creates an area of low pressure in the plenum, which draws air in through the front of the radiator while at less than highway speeds to balance the high pressure in front with the low pressure in back. If the plenum is not large enough (the shroud is too close to the radiator) there is not enough airflow to pull the air through the areas where the shroud is too close to the radiator, so the only airflow you get via the fan is straight through the area directly in front of the fan. This also blocks ambient airflow through the rest of the radiator at highway speeds. This would seem to hold water in your case since now not only is your low speed and stop and go overheating problem still there, but the shroud is also blocking airflow at highway speeds since it has blocked all airflow through the areas where the shroud is too close to the radiator.

Put another simpler way, there are two ways to block airflow through a radiator - you could put a piece of cardboard in front of it., or you could mount the same piece of cardboard tightly behind it. Either way, you cut off airflow through the radiator. In effect you have mounted a piece of cardboard with a 16" hole in it behind your radiator. The shroud needs to be bigger with regard to depth, creating a plenum for air to move. Sorry for the rambling, writing is not my forte, hopefully you get the concept of what I am trying to say. Good luck.
Doug