As hoped, the spark plugs were the cause of my problem. They were fouled pretty badly, but just re-seating the plug wire boots seemed to help; I didn't have a long socket that fit into the spark plug wells, so had to put the wires back to make a run to a local Pep Boys, and reseating them made a noticable difference in spark. Once installed, I was back to boosting at 16-18 psi. Not bad for $9 worth of plugs and a socket.