Heater core and box

[JamesM]

I've already been through 5 heater cores on my '93 Chevy Suburban in the last 2 1/2 years. The dealer shop is honoring the warrenty on the one they installed this June, as they are installing the second replacement for that one (the other replaced in Sept.). Now, they say the heater box (containing the heater core and costing $163.00) is cracked, and that is probably why the cores keep failing. Does this sound right?

I literally had most of the contents of my radiator pouring out the passenger door when I pulled into the shop!

What else could be the cause of this problem? This burb has a history of coolant problems, including radiator leaks in the original, a radiator replacement (the 5-year-old radiator is now seeping), and replaced the "nipples" at the back of the intake mainfold twice.

(Lemonade anyone -- with a green-ish sweetner?)

Please help.