[midgetracer]

it looks like silicate drop out. Are you using the Anti freeze that is made for modern aluminum engines. they put in large amounts of silicates to prevent corrosion of the aluminum. Also, the pH of the antifreeze is about neutral. The old versions of antifreeze had very little Silicates and ran at a high pH. When Aluminum engines arrived, this antifreeze would corrode the aluminum( look at aluminum heads on an old flathead). The next answer was to drop the pH to slightly acidic levels. This saved the Aluminum, but corroded the cast iron. As a compromise, lots of silicates were added and pH was kept neutral. Some industrial antifreezes are low silicate and work well for our old engines. The other answer is to change the antifreeze often.