Immobilizer or something else?

My 2002 1.6 Octavia started giving me some problems couple of months ago. Once or twice with couple of weeks apart it wouldn´t start.

Turned the key, the immobilizer light would light up for a second and turn off, starter tried to get the engine running but nothing happened.

That day I didn´t have time to troubleshoot at that moment so when I tried again later that day it started normally.

This weeked the car had been running fine for weeks but when I started the car the engine would run for 5 sec and then stop.

In between my attempts to start the car it either would not start at all or run for 1-5 seconds and then stop.

Tried both keys. Same thing. The immobilizer light lights up for couple of seconds and turns off.

I´ve read through this forum and tried to pinpoint what the cause is but still not sure.