f150 idle problems

My f150 when you crank it up at a normal sitting temperature it idles at 2000 rpm for 5 seconds or so and then sort of bogs down. Crank it
back up and it does the same thing but it doesnt cut off it just bogs down and then the rpm's jump back up. It repeats jumping around 4 or 5 times until it idles at about 800 then its good to go. It's an 89' f150 with a new 302 fuel injected.
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