Sounds like fuel delivery. Not sure what controls the 1990 has but it's probably flooding. Have you checked the color of the plugs.

Poor cold starts with good hot running can be a bad ECT sensor. Badly leaking injectors will show up more with less air (light throttle) but get better with more open throttle. Malfunctioning MAP or MAF(dirty) will give computer wrong air volume info. Vacuum leak will be worse at low rpm but better at higher rpm.

Good under load running does not indicate blocked exhaust or fuel restriction.

92 Montero LS 3.0L V6 Auto, Stock, Original owner, 185,800K miles