Reach back and feel the large vacuum hose located on the passenger side near the firewall. If squishy yank it off and inspect for a slit or leaks. I replaced mine with 3/8 fuel hose.
Just a couple of days ago, my 4.2 started running poorly, rough stumbling idle, surging idle, near stalling at low load, low rpm, discovered that the vacuum hose that plugs into the back of the upper intake manifold had somehow popped off. Couldn't see it was off, had to reach back there and check. Was guzzling gas too.
So nice when the fix is free for a change. Good luck.
1999 f150 xl - sclb 4.2 auto 3.55 330k km
LT265/75/16 tires, Hellwig overload springs, KYB Monomax shocks, v8 radiator