Spontaneous XP reboots on XPS-720

I have several XPS-710s, XPS-720s, XPS-730s, and T7400s.  When I run a test program that starts multiple threads, each manipulating a large memory-resident linked list in non-paged memory, all the while doing extensive floating-point operations and writing bitmaps to three open windows--when I run this test on the 710s, the 730s, or the T7400s, the program loads the processors to 100% and will run for days without issue until I finally close them out.  When I run it on any of the 4 XPS-720s, though, the program induces a spontaneous Windows reboot after 30 to 120 minutes--no Blues Screen, no bailing application, no Event Log entry either in the bios log nor the Windows log: the screens just go blank and Windows reboots.  This happens every time on the 720s.  According to Microsoft, spontaneous reboots only occur when the hardware fails, yet the Chat Tech tells me that unless I can identify the failing hardware component, the problem is mine, even though the machine is still under warrantee.  They say that since their Diagnostics pass--you know, the one that has you hit all the keys on the keyboard and look at the colors on the monitors and hits the hard drive for 15 hours--that since their Diagnostic program passes, it can't be a hardware failure.  I've worked with Dell support on this since September of last year.  On the first two 720s--the last 720s I purchased--Dell finally gave up and replaced them with refurbished XPS-730s--and the problem went away--but not before replacing the motherboard and processor several times, the H2CR cooling unit twice, and the power supply twice.  Which is alright--they did good.  Of the two remaining 720s, one is off warantee (I use it as a door stop), and the other, which is still on warrantee, Dell refuses to fix, although to be fair they are willing to fix it if I can identify the failing component (wait--isn't that the job of Tech Support?)