ground/earth problem.

hello , I hope all is well.

i made a relays and led circuit recently which a micro controller (arduino) triggers via its logic 5v outs. I built and tested the circuit on a work top table using a standard 5v power supply to test and trigger the relay/led circuits (to save me booting up my pc+arduino each day) and a 12v supply for the relay/led circuit.

The 12v relay/led circuit works great, but when i finally replaced the 5v psu and connected the arduino, the earth's/grounds of the circuit and arduino just do not like each other..So I installed an opto-isolator circuit between each and wow it all works !! great, but the problem is that when i installed it all into a metal chassis the usb plug and the power plugs both touch the chassis and cause the same problem as before, so what i did was use insulation tape around the usb connector, this just seems so bad and not a good idea.

i think its a ground loop problem potential difference or something? i just hope someone might know of a fix that will work? I have no schematic because my circuit is so hard to see now under all the hook up wires. if anyone might know a fix i'd be so grateful

thanks for reading.
harps : )

I would have to see a schematic to get a clue of what is going on.