I am really struggling to find a solution for my officeserv 7030's that keeps on randomly rebooting.

I have bought 3 7030's in the process of trying to solve my issue.
The problem is that my 7030 would just randomly reboot for no apparent reason. I have programmed all 3 my systems exactly the same and no matter which one of the 3 systems I use, it randomly reboots.

I have tried the following to solve the issue, but without luck:
- Unplug the network cable - system still reboots randomly
- Place behind firewall- system still reboots randomly
- Upgrade software to version V4.75 and then later to 4.91 - still reboots randomly
- Replace all cards such as BRI cards etc on the motherboard - system still reboots randomly
- Add battery backup - system still reboots randomly
- isolate on hold music input with audio transformer for groundloops - system still reboots randomly
- Check my programming for any issues - cannot find anything specific that I think could cause the rebooting
- unplug phones 1 by 1 to see if they are causing the problem - but system still reboots randomly

My next step would be to try and upgrade the bootroms of all the systems. I have noticed that often when the system reboots, it just sits in an unresponsive state and does not come back on, so I have to cycle the power a couple of times to get it back on again.

Does anybody have any ideas as to what I might be missing?
This is a really frustrating problem for me.