intake leaking ?

i have a 1993 silverado with a 350. About 3 weeks ago it developed a miss. when hot. I know my way around and engine compartment but this has me stumpped.Its has a complete tune up.From the test i ran it seem to be the intake leaking . I took a compression test all good. did a leak down on all cly. all good.did not have a raditor pump to pressure test system. No water in oil. Pulled intake and got one from juck yard it looked really good . install ed and have the same problem. one more thing when its starts missing the oil pteddure drops to 10#. it goes up and down from 10-30-#. Any help would be deepley appreicated