Nikon F3 / SB-16A flash problem

Some of the frames on my last two rolls of film have a problem. Half the
frame is exposed fine, and half is overexposed to the point where the prints
on that half are almost (but not completely) black. It only happens with
shots using the flash, but not all of them. Even when taking flash shots in
succession, one may be fine, but the next one isn't. The dark band is
vertical and covers just less than half of the left part of the frame.

I bought the flash just under a year ago from B&H (I actually bought it as an
SB-16B to replace an older SB-16A, and gave the old flash and new base to my
sig other to use with an N2000.) I will of course replace the camera
battery and clean the flash contacts, but has anyone out there seen anything
like this before?

Thanks! -- ric