1995 Chevy Blazer idle

Blazer idles very high, almost rips the tranny out if you put into drive. Doesn't always do it though, when it's not idling high it idles rough and backfires sometimes and want's to stall and then sometimes normal. When idling high I have to shut it off and turn back on several times until it's okay to put into drive, tranny is slow to shift if idling high. Have replaced 3 coil packs ran okay for awhile but doesn't seem to help this time. Also have replaced cam shaft sensor, idle air control sensor, plugs, wires, computer, throttle position sensor, air mass flow sensor. Has been in 3 garages several times and no one seems to be able to figure it out, pretty much paying for nothing getting expensive. Would appriciate any new ideas. Thank you