The 6P has 2MB. The 4L has only 1MB, but it's a 300dpi printer, and I was printing at 600dpi on the 6P. I'll try 300dpi and see if that does it. I assumed if it had enough memory to print a page, it would still have it in memory, but maybe not.
I have a bunch of old memory lying around, including the right SIMMs for the 6P, but for some reason none of what I have will work in it.
600 dpi needs 4x the memory of 300 (300x300=90000; 600x600=360000; 360000/90000=4). Sounds like memory may be the issue.