Intake leak or not?? Long story!!

I have an 87 5.7 tbi which puzzles me . When I start it after it sits for about 8 hours the idle raises up and down for about a minute then levels out. There seems to be a random miss coming from both tailpipes ( true dual exhaust) but truck has been been fully tuned up, all sensors check out or have been replaced. Upon startup it will puff blue smoke once from the pipes then no more unless engine is up to operating temp and I sit for a couple minutes at a light or idle for a while in which it puffs a nice cloud out the drivers pipe upon takeoff . I have been told that is valve seals causing that although I pulled all the plugs and none are oil fouled or covered. I have hooked an inline spark checker to each cylinder and they are receiving proper spark. My snapon scanner is reading 8-9hg of vaccum which from what I gather is low but it reads steady. From what I have been told this could be caused by intake gaskets or manifold leaking and causes poor idle and poor performance. I would hate to pull the manifold and find nothing wrong underneath but it is driving me nuts. Also would a leaking manifold gasket cause oil to be drawn into the cylinders or is it a safe bet I need to do valve seals while I'm at it? Sorry this so long but I do appreciate any help!!!

valve seats are shot. very typical on this motor. my 88 does it every day.