As Jan has mentioned, it seems like your cpu is 'latching up' and no amount of resetting will recover it. Therefore it seems you are suffering EMI problems to the extreme. As also mentioned, optos only galvanically isolate the circuitry - they have capacitance so fast transients will pass on through.

I doubt there is one solution to your problem - it will most likely be a number of strategies will need to be employed. Firstly, you mention high current DC is being switched - is it snubbed? Some R & C or a varistor across the relay contacts will go a very long way to removing fast transients from this source. I recently had a problem where the transients weren't affecting my electronics but were causing a usb to serial interface to lock up - and it was opto-isolated!

The other thing you need to look carefully at is the routing of the power and in and outputs of your board - make sure there not running in parallel with other signals. Frequently I see people loom the control wiring alongside the mains and switched circuits. Whilst not technically illegal in most countries in regards to the wiring rules, it is bad practice due to coupling.