Well, she's running much better with the new manifold gasket, but still not without problems. Going to check the valves next. I'm also going to check all of the vacuum connections that sprout from the intake manifold - looks like one goes down to tranny, another to the brake vacuum assist, and a third to the "peripheral" vacuum system (i.e. the blue lines). I haven't heard any hissing within the cabin, but I'm going to check all of the connections anyway. I don't think a minor leak in this system would cause noticeable problems, but you never know. Does the tranny shift from vacuum or mechanical linkage? A final thought - what are the chances of a partially clogged exhaust system contributing to this problem?