it seems to me the whole problem may be due to contacts....
i have just gone through a similar issue...car fires up perfectly for 2 secs & dies...but no light remains lit - no car key light, no injectors, no engine management unit, nothing.
removed cleaned refitted the relays in front of the battery,
removed the immobiliser unit, opened the case, inspected the PCB (for damaged caps or other components), refitted.
removed the fuse holder unit & all the connectors, refitted everything.
removed the RF unit from around the lock, tested its operation by using a spare key against it while using the original key in the lock...saw that it operated fine (gave the same fire for 2 secs symptoms when i didn't start with a key in close proximity to it)....refitted the unit.
now all seems to be working as per normal...so now, what was the real problem?????