While I agree that it is good to fix the radiator shroud - I do not think this is your issue, especially since it has been like that for a while. If you have a working fan, and enough water flow I don't think you would have an issue in the state you have it.
I have run cars, granted, not LCs with no shroud at all with electric fans because of clearance (engine swap in a CJ7 and a YJ), and for a while I had the fan straight wired for trail rides and I could not get it above 180 not matter what I did.
This seems like an air flow issue, but it could be related to engine RPM - have you checked the belt/pulley? Could the belt be worn or under-tensioned and slipping so you are not getting full engagement? Could the impeller on the water pump be worn? The higher RPMs may be allowing them to do enough until you hit lower RPMs...and the symptoms would present in a very similar manner.
Just another idea since you are scratching your head. Hope you get it figured out!
I have run cars, granted, not LCs with no shroud at all with electric fans because of clearance (engine swap in a CJ7 and a YJ), and for a while I had the fan straight wired for trail rides and I could not get it above 180 not matter what I did.
This seems like an air flow issue, but it could be related to engine RPM - have you checked the belt/pulley? Could the belt be worn or under-tensioned and slipping so you are not getting full engagement? Could the impeller on the water pump be worn? The higher RPMs may be allowing them to do enough until you hit lower RPMs...and the symptoms would present in a very similar manner.
Just another idea since you are scratching your head. Hope you get it figured out!