Why is my bronco insanely fast sometimes after I start it and then not others?

What is your question?

I have a 1990 5.0l bronco five speed, sometimes when I start it shell fucking fly like a mustang and make it to 6000 rpm with ease and and then others she's slow as shit and doesn't like to go beyond 4000 rpm, any ideas why it does this? I'd really like her to run fast always otherwise she has no power.