Hi folks, a newbie here

I have an 96 F250 with the PS engine and it has about 280,000kms on it. I'm on my third transmission

and I've had a number of igniter's replaced and the normal filters and oil changes, but aside from that the engine has never had a wrench on it.
Nothing I can really put my finger on but I feel that the engine is not putting out the power it once did. I'm not too interested in putting on a lot of after market enhancers but I would be like to take it into a reputable powerstroke mechanic and have him do a complete engine tune up. I have know idea what the tune up would involve and I'd appreciated anyone that does know filling me in
Thanks!