You will need to provide more details. There is a number of reasons this board might fail.
- Cheap relays sometimes get stuck in one position (because some glue or sealant leaked or the contacts get fused together)
- The USB communication might get out of sync and disconnect the board (from the price I assume it's using V-USB or similar which doesn't work perfectly)
- There might be a bug in the software
- Some channels might be overdriven and cause the micro to fail (too much current on a pin)
- ...
When the failiure happens check whether...
-... the PC still recognized the USB device
-... shaking the relays gets the relays unstuck (BE VERY CAREFULL NOT TO TUCH THE LIVE CONTACTS!)
-... wrong relay contacts are fused together
-... the build in serial is able to toggle the relays
-... the fuse or chips get gets hot
-...
That's all I can think of on top of my head. Use your's now!
BTW: While at the topic of relay boards, does anyone know if there is a chip similar to the
ULN2803 (multi-channel darlington driver with fly-back diode) that is also opto-isolated and not too expensive?