1987 Chevy Blazer Power problem

This is an automatic with a V6, 2.8 liter engine! This is an off again, on again problem! Upon start-up, it will backfire and have no power, it will run but its power is very erratic. Now if I take it up on the freeway and crank it up to 65 to 75 mph for say 10 to 15 minutes, it will cause all the power problems to disappear. Also upon start-up, if I spray carburetor cleaner into the throat of the carburetor while it is in idle the aforementioned problems will also disappear. The rest of the day it wil run very good, but in the morning all of the bad power probelm will return! Now what is wrong? My brother thinks that it may be a vaccuum leak! I have also used additives to add to the gas tank to clean out any water or other debri that may be in the gas tank. Thanks Joe