You may not like this but I have a 2004 3.0 that I have just sent the heads to get a valve job. 116,000 mi. Mine has been doing the same as yours for the past few months and finally I figured I better fix it before I ruin my engine. I could keep the CEL from coming on at stop lights and such by keeping the RPM about 1500. But you can`t do that forever.
The exhaust valves had worked themselves into the exhaust seats about 1/64th inch or so below the top of the valve seat. That is a LOT of wear. I figure Ford had a very good reason to put softer seats in these heads but hey that`s just me thinking out loud. Since my valves were now closing later there was evidence of back-flow into each combustion chamber coming from the exhaust manifold.
Anyway, If I was you I would run a compression check. I had 190-210 on 4 cylinders and like 180 and 150 on the other two. When I took the heads off all the exhaust valves had worked themselves down into the seats.
I checked my lifters for wear thinking the pickup might have gotten hot at some point but no, the lifters look almost brand new. In fact (I bought the truck used) everything except those valve seats look almost brand new. Something to think about.
If you do it soon you might head off a lot of expensive repairs later. It is going to take some labor though so get your mind right. But it isn`t that bad. Anyway, I can`t wait to get my heads back from the machine shop tomorrow. I miss my pickup.
Anyway, good luck.