A compression check is in order. What year model is the engine and how many miles? If it is pre '73, has it had lead substitute to keep the valves from sinking? Have you checked valve lash yet? Could be anything from timing chain having jumped, a few exhaust valves not seating, or maybe a crack in the bottom of the intake manifold sucking exhaust gas into the engine (My last \6 came afflicted in said manner). Or vacuum leak, as mentioned by others.
