Radiator Flush question...

[atdog]

I checked some threads but really didn't see anything that matched what I'm needing.
When I got the car in April (1974 H/O 350 ), I did a major flush of the radiator.
It was running hot at the time. Took out the thermostat, replaced clutch fan, and did the major flush. I'm not running A/C at this time (bad compressor).
The flush improved it, but it still must have some blockage ( the car wasn;t driven hardly at all for close to 8 years ), my guess is mineral deposits.
It still runs about 210 when driving 55mph, less than that at slower speeds, BUT when I run 65 to 90 mph the temp gage climbs up there.
Last week I was running close to 90mph on the interstate and got up to about 132 and I soon as I got off the interstate and did 55mph it dropped back down to 210 rather quickly. It also gets to 210 pretty quickly if I do 55 mph after about 3-4 miles.
To me anyway, that sounds like a blockage even though it flows well when I check it at idle.
I looked all over the place for some phosphorus acid or some product that might have it as an ingredient, but no one will sell it to me. New gov. restrictions strengthening the gene pool I guess. The stuff at the autoparts places is weak for my application.
An old timer mentioned something about running a cycle through the cooling system with some Liquid Plumber, but that didn't sound right, although I did use a small amount of LimeAway when I originally flushed it.
Any shade tree chemists out there who can suggest something that I can buy over the counter?
I was just going to take it to a radiator shop, but I'm running out of time before Dalton. And I don't want to drive to Georgia going 60mph :)

[jim77]

Did you pull the block drains? If you do see what comes out when I worked for a shop I had a customer with a olds wagon that had a similar problem. I pulled block drains and had to force a screwdriver in to break up rusty sludge flushed water in and the muck that came out was unbelievable I also put a radiator in and car never overheated again. Do you have a fan shroud on car that will cause overheat situation in some cases when it has been removed. Good luck let me know what you find first thing I did when I got my car.

[Rocket 350]

I've had good luck with CLR in crusty radiators. Drain coolant and refill with a plain water and a bottle of CLR. Start up and take for a quick spin or just let it idle for about 10 minutes after the top radiator hose gets hot. Just make sure not to go for more than 10 minutes or so before you drain it or the acid will start to attack the aluminum. Follow that with plain water and a bottle of Prestone flush or a cup of dishwasher soap and a nice long drive. Drain that and do several plain water flushes. Refill with 50% coolant / 50% distilled water. Don't forget to pull the block plugs also everytime you drain so you get it all out.

[atdog]

Update:
I found some sulfuric acid yesterday and spent the whole day flushing.
Prestone flush, water flush, acid flush( that stuff was cool to watch work! ), baking soda flush to neutalize the acid, another prestone flush, and more water flushes....it's still doing the same thing. I'm going to take it out and take to a radiator shop to have them rod it out. It has definitley improved the radiator flow, but not enough. After about 15 various flushes, I'm flushed out.
I'll just watch my speed until I get it to the shop.