Mega 1280 Home automation lockup after power reset

Hi Guys I’ve got a home automation setup using a Mega (1280, DFrobot) Ethernet shield, web interface etc. Its running fine for weeks at a time then every few months it locks up and wont restart, here is the strange part, even if I remove the power from the mega and replace it, it will not restart the sketch, the mega freezes soon after starting after the boot loader.

If I press the reset button on the Mega board … it starts then locks up as soon as the boot loader delay is over
If I press the reset button on the Ethernet shield … it starts then locks up as soon as the boot loader delay is over.
If I unplug power and leave it for a few hours then re-apply power … it starts then locks up as soon as the boot loader delay is over

The only way I can restore full functionality is to connect my laptop to it and re-upload the sketch to the mega, which will then work as expected for a few months. Until it locks up randomly again.

My sketch has a web front end that I’m using to set heating values etc. and it also tells me the available memory (sram) on the mega which is usually sitting about 2-3k. so I don't think its running out of ram

I’ve just about done my head in looking for a cause to the lockups.

Any Ideas...

Regards
Craig Whitto