Here's one more for you. When I got my Tiger after 17 years of storage, the original owner gave me some history. Except on the coldest days, she could not keep it from overheating. Despite numerous tries at fixes (heater core cover removed; heater control valve locked to open; correct six blade fan, etc.) she said she had been unable to correct the overheating problem.
Well in putting the car on the road, I pulled the radiator to have it boiled after so many years. The radiator shop made an amazing discovery. While the car itself was rust free, and the radiator pressure checked fine, the fins had actually detiorated away from the tubing in the Malibu air. The radiator, which to all outward appearances was fine, was recored. Since then, it operates at 172 deg no matter how long I idle, where I drive, or the load on the car. This is pretty amazing as I am in the Mojave Desert. It was also a relatively cheap fix.
Good luck.
Bob.