Friend had a similar problem in a Jeep. We flushed the radiator, engine, and heater core several times but the radiator ended up still being clogged. It allowed just enough coolant to flow to keep the motor cool for a little while. Then all of a sudden the temp gauge would go up like a rocket. We ended up getting another radiator and it ran fine.
We got a radiator for a really good price
here if that ends up being the problem.
Good luck.
