HA setup (active/passive) - Slave keeps restarting

I searched through the forum, but could not find the same behaviour, so here goes:

I have an older box running 8.305 which is end-of-life or about so...
We purchased another machine (QC Xeon, 8GB Ram, RAID-1 disks, 4xGB, ...) and I wanted to run those two (for now) in Active/Passive, so the transition (when it comes) is done as smoothly as possible...

Now, the configuration of the HA is no issue, the 'Master' being the old box, the 'Slave' the new, all that went OK, and the two boxes see each other and replicate.
BUT....
When the sync is done and the Slave becomes ready, the box (the new one) restarts, and after reboot, starts syncing again.
It has been doing this for half a day now... Tried everything.

I switched LAN ports on both (accordingly) where the 'HA sync' interface was previously eth3 and is now eth1, put in the 'backup Sync interface' through our internal network, checked and replaced cabling where possible (all cables on the new box at least), but still no resolution.

The new box restarts, and a 'System Restarted' mail message was sent with reason 'unknown'... I tried following what happens in the logs, but nothing I could find points towards a cause or solution. I hooked up an LCD to the new machine and nothing is displayed right before the restart that can give any clue on why it does that...

Any ideas where to look or start checking?
I'm all out of ideas now... :(

Thanks!