Doors and Windows - Door frame sagging & shifting

My front door (Caradco, wooden, 5 years old) is starting to sag. When standing in the front yard looking at the door, you can see the wood above the door slightly sagging.

How the heck can I fix this? This door has also shifted quite a bit over the years so I had to trim the bottom to get the door to close. So, at the top of the door (knob side) there is a 5/8" or more gap, but at the top of the door (hinge side) there is only a 1/8" gap.

Any ideas? :)

Since you said "door" I assume it's a 3' wide door--the only things I can think of to cause this is either the door was not properly attached to the house framing, or something under the sill is rotted away or otherwise sunk into the ground. Is this 5yr old construction, or a replacement door? May try placing curved bar under knob side and prying up to see if any give--if not the I would tend to think decay in floor joists and plates. If the door edge raises by any appreciatable amount, then it is not screwed correctly to the hinge stud/trimmer.
Hope this is of any help.
chuck

Yes, it's a 3' replacement door on a 30 y.o. house. From what I can tell, the door frame is sitting on top of a joist header which is on top of the foundation. I did not notice any rot. But, I do have some settling problems with the foundation (several hairline cracks in various parts of the house drywall). How can I tell if it was not properly attached? I mean, should I be able to see filled nail holes along the door jamb or something?

About the sagging...you actually can see that the center top of the door frame is about 1/2" lower than top two edges (hinge side & knob side) of the door frame. I am unable to press this up to correct it (or even budge it)...of course, I hadn't tried a jacking stand, just a little muscle. ;) Whatdyathink?

If you have sagging in the middle of the door frame then the header is either not there or not broke, so the weight of whatever is above is pressing down on the door.