Toyota Repair: 84 Cressida -- cooling system, temp guage, liquid aluminum

Question
Hi Paul,  I have a 1984 Toy Cressida.  Wonderful car, most of the time.  Here it is.  Two weeks ago I decided to put some liquid Aluminum in the system for preventative maintenance.  Same day the car began to try closing in on overheating.  Coincidence?  Of course it was.  I've used that stuff in many cars.  Anyway, I checked the waterpump belt and it's in good condition with proper tension.  I checked the thermostat and it was BAD.  I tested it in boiling water, and it barely opened.  I replaced it with a new 195 degree unit.  I tested it also before installation, and it worked fine.  Also, I did my best to flush the system as thoroughly as possible.  I even checked the vacuum controlled heater valve off the car.  All systems OK.  Put new fluid in and ran the car.  Seemed OK driving around town.  Note: It has been somewhat cool outside.  Anyway, the temp guage was running in the normal range--around town.  Then I had to use the freeway.  At high speed (55+), the cooling system trys again to overheat.  I had to use the heater system on high fan with the windows partly down to help keep the engine temp below the red zone.  Other data includes the fact that at "low" car speeds the engine temp will climb with the AC on.  The engine temp seems to behave below 40 mph without the AC on.  Remember, days here are not very warm at all right now.  Other facts include: Water pump is NEW, and the fan clutch is NEW.  NO LEAKS. Proper anti-freeze and water mix.  The only original thing left is the radiator.  Do you believe that the radiator has become incapable of cooling the system in warmer temperatures?  And that this could have developed over the winter when it would not have been noticed?  I have had this car for three years, and it has never heated in an abnormal way no matter the ambient temperature, no matter what accessories are used.  The radiator does seem to have a degree of whitish build-up inside, but not enough to restrict water from flushing through the core--so it seems at least. (I found a new radiator for $118 at Express Auto Parts on the internet.)  More information: Timing seems OK.  Alternator NEW.  AC pump turning freely when not ingaged or ingaged.  So, do you think the radiator is the most probable cause, should I try to get it fixed or just get a new one.  If you have other ideas, I would sure appreciate your brain power here.  This probem of excessive heating DID all start when I put that Aluminum stuff in the cooling system.  I don't think it could have finished off my radiator, do you?  Well, thanks for being out there to help approach this kind of thing.

Answer
you might as well put the new radiator in at this point. i never recommend adding anything to the coolant system such as stop-leak i.e. then i would top it off with fresh fluid. try that. i hop i helped.. it seems at though the radiator is restricted.