I have a 2001 Expy EB 4X4 5.4l with a little over 160000 miles on it and im having some problems with the lack of low end power, the trucks idles a little rough but not bad. mostly just a mild engine shake. I have noticed that when i start the truck the RPM's go up to about 1200 the idle down to 1000 and slowly bounce dropping to 750 and set there for a several seconds then idles back up to 1000 then back down. Now when i gas it a little while in park it does the same thing except it doesnt jump back up once it goes down to 750, also the truck doesnt drop any RPM's when i put it drive either. Now back the lack of power. it feels like something is holding the truck back when i take off from a stop or accelerate up hill or from a slower speed. it seems to run fine when im on the highway. you cant feel the power until the rpm's get up to around 3500 rpm's.
The mods that i have done is the rear 2 cats removed flowmaster 40 series SI/SO muffler in place of the stock one, and a k&n filter with the gotts mod. So far i have changed the plugs to NGK platium and checked the PVC tube and elbows cleaned the MAF, IAC and Throttle body.

when i do floor the truck from a slow speed the engine does rev up, but it want to take off till its hits about 3500 rpm's then she really moves.
now i have had someone tell me to try and power brake it to see if it is missing when under a load, and when i did that the transmission started leaking at the bell housing, and i couldnt get anymore than 2500 rpm's while power braking it. Im at a loss, i cant figure out whats wrong with it.