Mail merge print problems

[golfpro]

I am doing a mail merge using an excel spread sheet to put addresses on a publisher post card. I am printing them 4 up, flipping on short edge, using a dell 3100cn printer with duplex module. When printing, the first 8 cards (2 pages) will print correctly. only the front side of the next page will print -the back is blank, the forth and fifth page will print correctly, the back of the sixth page be blank. So, every third page will have a blank back side. Any ideas on how to fix this?