Try changing the PCV valve.

 

i have had my 99 4.6 since new (120k mi).

the 4.6 has a good rep however my 4.6 had mult probs (warped head, etc) early on, but then got 100k prob free.

when i was buying the truck research showed the 5.4 to be very problematic.
i know of several persons who bought the 5.4 in '99 and had to have the motor replaced (one guy twice).

not sure how that factors into your buying equation but you should not compare the '99 4.6 to a later 5.4 if you are thinking of buying a '99. not sure when the 5.4 got better, but it is def a great motor now.

one more thing. do not use arco gas, you will see gunk buildup and high idle (about 100k). switched fuel and prob disappeared.

 

Get a 300 I6.

Tis a joke. Just thought I'd throw that in the pile.

Givin the choice, I would go for the 5.4