I recently flash my Dell streak from Froyo to Gingerbread and using Longhorn custom ROM. After awhile using it, i notice that it randomly reboots without no specific cause. I did some digging and apparently many of those out there are facing the same issues on various ROM.
Good news is, _n0p_ from XDA developed DSC ROM and found the fix. It is apparently and issue with the kernel and with baseband switching 3G modes which cause a reboot command to be sent.
To find out how to fix the issue, please head down to this post and look for the ***Updated 2013 Feb 26 *** section.