Sb-600 not beeping, images completely black...

hi, i use a d200 and have linked my sb-600 as a remote to my sb-800, which is on camera. when i am shooting a series of images, the sb-600 will at random stop giving the 'fired properly' beep; it still fires, but no beep follows and all the images from there on out are completely black and underexposed. The only thing I do that makes it come back is shutting everything off for a few minutes, then turning back on...I have checked the sensors and line of sight, no problems there -- this is happening at completely random times, when all ready lights are on and nothing has been repositioned or settings changed...please help!!!!

thanks!!