I just went through this on mine. My transmission ended up needing a rebuild (not cheap, by the way), but I learned a couple things along the way. Since your fluid is clean, I suspect it's overheating and kicking into "limp mode" which very much feels like it's slipping. Seemingly, a very common cause for the transmission overheating is the check valve in the transmission line. The first thing I'd do is remove it, and replace it with HIGH PRESSURE 3/8" hose. The attached shows you where it is. I didn't get so elaborate when I removed mine; just cut the pre-made line where it comes out of the radiator and put the new hose in with some good clamps. If you research the subject you'll realize that just how common the problem is...

http://dodgeforum.com/forum/2nd-gen-ram/182742-trans-check-valve-delete.html