Good call @Ilivetoski, how I check for vacuum leaks, keep in mind I am a farmer and a hot rod builder, somewhat redneck too, is with the engine comp open run it to the point of bog, spray a bit of ether or WD 40 around the intake runners/throttle body, if you Hear the bog go away or the engine fire properly, you know you have a vacuum leak. That being said you have to be fairly precise to not start a fire... Beings as it starts doing so after its good and warm (expansion) makes me lean towards a vacuum prob.