cat 311 excavator not running smooth.

Just wondering if anyone has any ideas why the engine won't run as it should. Its a 1994 311 Cat with a Mitsubishi 79 hp. 4 cyl. it's got 6700 hrs on the machine. Engine starts and runs fine at idle but any throttle setting above that and it stumbles and runs pretty rough, seems to smooth out a bit when there's a load on it. I have tried everything I can think of but nothing has had any effect.

cat 311 excavator not running smooth.