Whats breaking my rad support?

[BenE]

I have a 71 C-30 that the rad support keeps breaking on. It breaks on all 4 corners of the opening for the radiator. Its not been in any major wrecks that I know of( not in the last 22 yrs at least),no noticable vibrations, rides and drives great. Has anyone seen or heard of a truck doing this? This will be the third rad support for this truck plus several weld jobs. Any ideas or suggestions will be appreciated.