if u get a new cat which u probally need one i would go with a high flow unit

 

Okay well I will look into a magnaflow cat or something.. do I need to use a special o2 sensors for the new cat if I replace that??

 

Try this first. http://dodgeram.info/tsb/1998/18-48-98/18-48-98-v8.htm

 

^^^ +1

Make sure the wires are routed correctly. And also check to see if your getting spark and each cylinder. I'd say chances are you have bad plugs in even though they are new. I had a problem like that with 3 random misfires and new plugs it and back to running perfect.
-Which brand plug did you put in?

Also, I doubt you have a bad cat cause when they go out they usually cause the rear plugs to misfire due to back pressure and heat

May as well pop in a new o2 precat sensor too and may help it run better and its cheap.

To see if you need to change the plenum gaasket pop off the air box and check down the butterflies in the TB to see if you have any oil panning in there...if so put the $125 Hughes plenum fix in. About a 6hr job

 

Remove the TB to check for oil pooling. You need a mirror and a flashlight so you can see all 4 corners and sides.