perpetual kernel panic / constant restarts

MacBook Pro5,3 (mid '09), with aftermarket 1TB 7200 HD (original fried), OS 10.10.2 at first had an unresponsive display upon opening lid / waking from sleep. Only holding power down for 10sec to reboot snapped it out.

Day later it started going into a constant reboot cycle, either with a 6-language message on a gray screen, or simply a Mac boot chord, followed by black apple on gray screen, and a progress bar that seems to only get to a third, before kernel panic occurs. It just reboots itself. Every 10th or so cycle, the OS actually boots up.

It has booted in Safe Boot more frequently. Even from recovery partition / bootable USB, only ever so sporadically. Verified Startup Disk - fine. Paired permissions. Same result. Reinstalled OS. Same problem. Ran Geekbench - awful results on read / write, and other HD related functions. Graphics and memory - average. After frequently getting to Safe Boot, decided to zero-write and clean install. Same problem. Problem seems to be tied in with "sleep" function. No Mac repair person has been able to isolate the issue, they are all puzzled and just say "get logic board replaced". In verbose mode, it most often hangs during the "fsck_hfs" or just after "Previous Shutdown Cause: 3", "DSMOS has arrived" and "Sandbox: " denying permissions, but not always, it's a different line almost every time. It says "removed 5 orphaned files", but sometimes 4 or 1 file. Other than shutdown cause: 3, there seems to be no consistency.

Thermal Sensor? Fans do blast like crazy.

Makes no sense to do a costly Logic Board swap on a 6 year old machine, does it?

Thanks!