I've done relay switching with ULN's too. I encountered no problems, and I took no SPECIAL precautions. (only proper grounding and a single 100nF). Everybody here seems stuck on this being a hardware problem. Nobody has ever considered if this might be a software problem (interrupt getting in the way or something). Zoojar, Are you convinced it is not a bug? Have you tried a real stupid testing program already to check if it is really a hardware problem? Your stupid testing program should do something like this:

reset:
turn the relays off
clear the LED
loop:
if the pushbutton is on, light the LED
toggle the relays
wait
jump loop

Run the program. The relays will go on and off and so on. Push the button shortly. The LED wil go on and stay on. Go drink some coffee. Come back and check. If the LED is off, the controller has reset, and it really is a hardware problem. Good luck!