"Bucking" in 3rd gear.

I have a 2003 Crown Vic P71 with 105.000 miles on it. It is a 4 speed automatic. In first and second gear, It runs smooth, when it goes in 3rd gear, It starts "Bucking" almost like it is running out of gas. 4th gear its fine again or if It drops back to first or second gear, It runs fine. 4the is also fine.
Do you think that it is an engine or a Transmission problem. I don't have a code reader.

Are throwing a CEL at all? If you are hit up your local O'Rielly or Autozone they can pull the codes for free.

My 99 had the same behavior just a little while ago mine ended up being a bad coil on plug and I had to change it out. But mine was doing it in all four gears. So to me this sounds more tranny related. One thing mentioned on www.crownvic.net for this issue is the fact that Ford transmissions can shudder under load with old fluid. They recommended a full flush and service and to add Lucas Oils Transfix to the tranny and that has pretty good reviews on the site. There is also a walk through on how to check the shudder in the link below.
http://www.crownvic.net/ubbthreads/ubbthreads.php?ubb=showflat&Number=2305433&page=1
post #2311411

my 94 had same same problem there is a tsb on this ,change all fluid,add cooler if not equipped,the fluid should be semi or full synthetic....it worked on mine gl