I have been having a similar problem with an MGB/GT. The engine is a fresh rebuild from a good shop. The radiator was rebuilt by the best shop in the area. The engine has a bit of cam, +20 pistons, a full balance job and brand new carbs. All hoses were replaced. I can drive it to work in the morning (5:30AM) without going over 180 F but when I drive home at about 6PM it is up to temp before I get to the freeway and over 200 F before I get home (7 miles total). In talking to the tech support people at Moss Moters, they suggested looking at the distributer. As the advance plate and springs wear the timing can advance radically over the original curve. They suggested checking the timing at idle with an advance timing light (one that has a dial that will allow you to dial back to zero and see haw many degrees advanced you are) at idle, 1000RPM, 2000RPM and 3000RPM. Also if using a vacume advance it can be tested by unpluging the hose from the carb and plugging the carb port. Next connect a hand vacume pump to the vacume advance host and gradually apply vacume to the specified spec for the distributor. Again use the advance timing lighi to see how many degrees of advance you are getting.
Also a hand held infrared thermometer is really handy (about $50 on so online) to chech temperatures. You can measure abive and below the theroostst to verify that it is opening at the correct temp. It can also check brake temp after a run and make sure that the turkey is at the correct temp! I have one on order myself.
