Hi,
I would bet dollars to donuts that the missing thermostat is the problem.
Without a thermostat, TRs move the coolant through the radiator too fast for proper cooling to take place. These cars should never be run without a thermostat (or similar restriction) installed.
Your statement that the problem is occuring more at highway speeds supports this: higher rpms move coolant even faster and the increased flow of air isn't solving the problem.
Install a thermostat. If you can't get an original-sytle sleeved one, likely at a ridiculous price, get a standard cheapie in the 160 to 180 degree range and restrict flow to the bypass hose. To do that, get a metal spacer or something that reduces the size of the bypass to about 1/4 or 3/8", by inserting it into the hose. Be sure it's a snug fit inside the hose.
Another way to achieve some limited bypass is to drill a small hole in the flange of the thermostat. Some thermostats come with this already done. Some bypass is important until the engine warms up and the thermostat opens to it's warm operating position, so I'd avoid completely blocking the bypass hose completely, although some folks seem to have done so and not yet had a problem.
By the way, if your water pump is turning freely and not leaking or making noise, I doubt that it's a problem. There are upgraded impellors, with 6 blades instead of 4, that racers use. However, this is not likely something you would need unless you regularly run the engine above 5500 or 6000 rpm. The special impellors are as much to prevent cavitation as to increase water flow, and are often matched up with a high volume radiator to be effective.
Also, pusher fans mounted on the front of the radiator are less efficient than puller fans mounted behind the radiator. I've seen some speculation that they might even hinder airflow. Of course, if you are still using the stock fan behind the radiator, for authenticity's sake, you don't have much choice. If you ever opt to move the fan behind the radiator, the original fan needs to be removed, but the hub assembly and heavy extension retained because it also provides dampening to the crankshaft.
Do you have radiator shrouding in place to direct air flow into the radiator? I think missing shrouding is more a cause of overheating in TR4s than TR3s, but it still might help.
I agree with other responses that too lean a mixture or incorrectly set timing might be part of the problem, too. Perhaps the car is running rich at lower speeds, but lean at higher rpms.
I also agree that a faster opening "Summer" thermostat doesn't really solve a cooling problem. It's just open sooner and will have lilttle or no effect on the cooling system, ultimately, once fully warmed up. An open thermostat is the same, no matter what it's initial operating range. The 180 or 190 degree will help the heater come up to temperature more quickly in the Winter, but that's about it.
I happen to have a 160 degree stat in my car, but just because it was the only size of original sleeve-style thermostat I could find when I was looking. I'd be just as happy with a 180 degree thermostat.
The primary reason to install an aluminum radiator is to reduce weight. Sure, some are built with many more channels than the original TR's 20, and might cool better. But, aluminum is less efficient than brass at conducting heat away from the coolant. So a larger radiator with more total cooling area is needed, to achieve the same level of cooling.
Does your radiator have the crank handle hole? If so, that reduces the radaitor's cooling area by a significant amount, 20% or more.
Hope this helps! Let us know.
Alan