On the Dorman intake manifold there is a L vacuum port on the back that you install (photo below on the one I removed from my truck). There is an O ring in the bag you must use to seal that up or you will have a vacuum leak. Since this is an O ring sealing system you can't use any RTV on the seals. The one I took off the old motor was a Dorman and I didn't know it until I'd bought the replacment. The installer used RTV on the water jacket area and that is likely why it was leaking. Also need to make sure that you have a pretty clean and smooth surface to install it. If there is pitting on the head it can give you a faulty seal.
Since you replaced the intake, did you also replace the O rings on the fuel injectors? You'd likely be leaking fuel if the top ones are bad and vacuum if the bottoms are bad. You could have a bad fuel injector. You could do an OHM's test on each to see if you have one that is out of spec. When I had mine apart I actually cleaned and serviced the injectors replacing the O rings, pintle caps and internal filter. Also you are working in inch pounds on a lot of this. I had no torque wrench that would go low enough to use reliably (and couldn't find one in town) so I pretty much went snug on everything. Since I had a Dorman on mine when I bought it I have nothing to reference how it performed stock.
I completely cleaned my throttle body. It was in really nasty shape with the throttle plate sticking in the bore due to gum build up. Excessive carbon that took alot of soaking to clean. I replaced the TPS, cleaned and reused the IAC (mine was fine and pretty clean). I cleaned the MAF sensor with MAF sensor cleaner, new air filter. The other thing that I noticed due to the age of the truck was rotting vacuum lines which I replaced and made the engine idle smoother. The multi vacuum tap on the passenger side of the throttle body was fine. I found two separate pieces of rotted rubber tubing and different junctions. The worst was the piece running from where the T goes into the firewall for the AC controls and to the vacuum reservoir. The other which had some rub through was the large vacuum line from the top back to the vacuum valve deal on the firewall that runs down to the brake booster. I installed a new PCV valve and checked the vacuum line that runs from the port I was talking about needing an O ring to the PCV valve. Check that line because it could get rubbed through. One thing that was almost completely gone on mine was the small peice of hose between the plastic tube and the intake hose coming off the drivers side valve cover.