rig keeps crashing

I have a rig with 4 r9 380x 4gb cards that keeps crashing. One of the cards is showing a lower hash rate than the others of about 14 MH/s. The other 3 run at about 20 MH/s.

I suspect the slow card may be the problem.

This time I restarted the rig with only the three faster cards and am now waiting to see what happens.

Has anyone run into this - a slow card crashing the rig? Return the card for replacement?

Thanks.