Not sure if the 4.6 has a vacuum pump like the 4.8 but my pump died (so slowly) and was giving me similar symptoms. When it got worse I'd hear a sound like a babbling brook behind the dash upon shutdown.

Hope this helped

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"When the Team Chief said.... You're trapped in a hole with nothing but a goat and a slinky, what do you do? Stubby said, I'm not sure but it won't end well for the goat...." ~(Overheard) Last day, Phase 3, Q Course