i had same prob with mine -98 svt -- and after in dealer and they had for -EVER- well figured out it was (white plastic clip)$3.00
plastic clipthat holds linkage in place (like say on normal doorlock linkage where a rod hooks into a round hole it has a piece holding it in place from falling out of hole)
-----anyway it was under throttle body,close to intake manifold and can be seen from drivers side front of engine- it apairently dry'ed cracked and fell out of place linkege was still inplace but there was enough slop that i could not feel that normal boost of power at aprox 4000 rpm--contact me if questions--sorry hard to explain but hope that it might help- that was the issue in my case.

silver frost 98' svt--tint, stereo system, kkm intake, glasspack, optima, carbon fiber, superchip, side markers, adj. koni's, eibach's, coilovers, axis 7evens 19x8, nitto 225-35-19