Two different boards with pretty much the same fault – some kind of horizontal glitch in all the sprites with blocks of 4 pixels appearing out of place.
At first I thought this would just be a simple addressing problem affecting the sprite ROMs but when I examined the data format in MAME each 16 pixel row of each tile is stored at a unique address (so the 16 pixels map directly to the 16 bits on the ROM data bus). So if the glitch is within a 16 pixel block it can’t be addressing, it has to be elsewhere. Unfortunately the problem wasn’t easy to find as the entire video board is for sprite processing. Eventually I found it – dead outputs on the PROMs at IC92 or IC94 can cause this.
Only one PROM of the pair was bad and it was replaced with one from a parts board. Problem solved.