If you have access to the equipment that is failling - do some tests to try and find out what is really causing the problem otherwise you might make a load of changes and still have a problem. It sounds like you can replicate the problem, so it would seem relatively easy to try some tests and see what the effect is.

first up, I'd get a small 15-40W switchmode psu and remove the 7805 regulator. At least then you should have a fairly clean supply that is immune to brownouts.


Then manually activate the relay circuits to see which one causes the most problem. Try some snubbing on the circuit causing this.

Run a separate power supply for the control circuits.


For the RS485, tranzorbs work well but go short circuit if your connect excess voltage to them (very easy if you have 24V floating around). The polyswitches (100mA) protect the tranzorbs so you can apply 24V to the RS485 all day and it will survive after you remove the fault. Or use LT1785 chips - beware though as they are slow!