'91 S-10 blazer idle problem

[likaroc13]

my '88 4.3 auto does something similar...when i first crank it (cold or warm), it idles up high for a few seconds...then it'll idle down to normal...i mean, i have to wait for it to idle down before i can put it into gear...i wasn't sure if this is normal either...my brother even asked me the other day if it was supposed to idle that high....but mine is fine after it idles down...the only other time is when you come to a stop in gear & under load, it vibrates really bad...but then it smooths out when you accelerate or even if in park/neutral