loose of power?

hi i have a 2.8 jeep diesel2005. it has lost power twice now,i just bought it. i have put 7000 miles on it since i bought.
you will be driving and you loose power ,but it still idles, but no codes. i have replaced the fuel filter and ebr valve.
it did again after replacing these components. to fix it all you do is cycle the key ,starting it up the first time it will still idle rough and then the next time it is okay and away you go,but no computer codes set.
so i am guessing it is not computer related and it is something mechanical that the computer does not control.
it is almost like the valves are sticking in the valve guides for a second and when you cycle the key they unstick?
engine has 80k on it. valves should have plenty of clearance by now?
run great and get 28 to 30 mpg.
george