this won't fix everything, but you would be surprised how often I run across this issue with such a simple fix

many LS400 years have three throttle cables, one for the main throttle, one for the transmission, and one for the cruise control

these throttle cables are all located near the TPS

please check these for tension, none of them should be tight, nor should they have excessive slop, Just a tiny bit of relaxed looseness you can feel with your finger, in other words, just a tiny bit of play or "slop"

if yours are not like this, you need a couple of 14mm and or 12mm open end wrenches and start adjusting!