Two pipe steam problem air locked or steam traps?

Roma

I have a trane two pipe steam system that has in wall trane convector boxes in each room.  My home is a 1936 tudor home approximately 2800 square feet plus an additional 600 square feet in the 3rd floor walk up attic.  I am burning about 275 gallons of oil a month (Yonkers, NY).  This seems excessive to me since the heat is set at 68 during the day (8am to 11pm) and 66 at night.  I recently took off the 2 main vents (hoffman vacuum # 76's) and the two return vents (hoffman vacuum #75's).  I replaced the vacuum vents with brand new Hoffman 75's and and boiled the other two 75 vents in vinegar for an hour and they seem to be blowing fine.  My steam traps were changed about 5 or 6 years ago (i have to check my records).  The boiler is a peerless about 17 years old.  The system also has a large trane bucket trap.  I recently had the asbestos abated as I am redoing the basement.  I replaced all the asbestos with 2inch and 1.5 inch fiberglass pipe wrap.  My problem is my system runs for a good 20 or so minutes but I only get about 5 minutes of heat thru my trane convector boxes.  When I go down to the basement and I check some of the still exposed small fittings they are still super hot but no more heat is coming thru the convectors.  Does this sound like I am air locked or trap problems?  Or any other ideas.  Sometimes the heat cycles on in a half hour other times and hour.  Yet the pipes downstairs are still very help.  How about your two cents.