1995 Other Mercury Models 95 merc Grand Marquis surges and h

Greetings. I've got a real stumper here, two pro mechanics have already tried and failed.

My 1995 Mercury Grand Marquis (V8) runs fine until, from time to time, the engine surges and hesitates. Sometimes sitting at stop lights the cur lurches forward, or driving (highway, streets, anywhere) the acceleration disappears almost completely, only to blast back intermittently. Using the gas pedal doesn't help, just have to ride it out. Premium gas seems to limit the occurrences a bit, but nothing has gotten rid of it yet. The fuel pump has been replaced, for instance, while the transmission has almost completely been ruled out as a suspect. One possibility is a vacuum seal, or so another mechanic told me when I described the symptoms. Spark plugs? I'm at my wit's end!

** Another note: I'd swear the cold weather also limited this problem. It does seem as though it's worse in the hot weather. That may or may not be relevant.