i know this is really hard to diagnose over the internet but here it goes:
car starts and idles fine but while driving at a constant speed the car will intermettenly start sputtering/bucking. when this happens my apexi turbo timer A/F reading will go from 14.7 to 20 then off the scale. however, when this happens if i give it alot throttle(3/4 or more) the A/F ratio will drop back down to normal and all sputtering will be gone. then after crusing for a little while it will do it again. i have allready replaced the tps with a new one an that did not help. the car pulls hard and runs well on boost. the problem is only during constant speed driving.

any ideas would be great.

list of mods:

s13 redtop
t3to4e
full race exaust mani
greddy type S blow off valve
greedy down pipe
apexi n1 exaust(single 3inch)
300zx maf
wolbro 255lph fuel pump
300zx fuel filter
740cc injectors
koyo radiator
fal twin cooling fans
blits LM intercooler
lightweight flywheel
spec stage 3 clutch(6 puck)
b&m shifter
apexi turbo timer
greddy profac b boost controler
various gauges
eibach springs
kyb agx shocks
apexi strut tower brace
j30 diff
motegi 16x7 wheels
adjustable fuel pressure regulator
tial 40mm waste gate
greddy pulleys(i thought they were underdrive)
megan racing streets (that sit to high even with the sliders removed)
greddy oil pan
arp head studs
greddy metal head gasket
nismo engine and trans mounts
apexi 300zx air filter
rom tuning

thank you for your help. this is beyond my skill level.

aaron