I would rule out a stress fracture...like I posted previously, doesn't seem that it's mis-mounted in any way...
For this to happen it would not have to be mis-mounted. It could be that the sub frame is flexing more than it was supposed to. Did you get a NEW radiator to replace the original or used? Is it a GM radiator or aftermarket? what type?
98 GTP Coupe- 180* T-stat, GMPP Front Sway Bar, Dual Aeroforce Interceptor's, TDC FWI, TDC Taylor 10.4's, ZZP SS Plog, ZZP 3" SS catted DP, ZZP MPS W/3.4, AL104's gapped @.055, 3800Perf. PCM