Idle probs

I've noticed when I start my truck up(98 4.0l v6) that itll fluctuate from 1200rpm to 600rpm, and it will do this until it gets warm enough to stay at 600rpm. Any ideas on why it does this?

Check your ETC. I wonder if it's 'shorting' showing a really (-10F) cold engine, then it improves when it gets a bit warmer. Is that truck equipped with an EGR system? My daughter's Taurus would rev up n down until it warmed up a bit, vac leak in the EGR system.

Idler air control, the cylinder thing on top of the throttle body. Its get dirty and sometime you can get away from cleaning it. I would just replace it.

Check your ETC. Is he shorted?