My 6.5l diesel started "missing" now it has hardly any power at all, any suggestions?

I have a 1999 chevy with a 6.5L diesel motor in it. Just recently I noticed that when I would take off down the road, the vehicle would jerk from time to time(two days ago). Today I went to start it up and it started hard. I took off down the road and could barely get it to go 55, it also took forever to get it up to that speed. When I got back home, I shut it off and restarted it. This time the motor started more easily, so I took it back out on the road. It had it's power back but the motor was sputtering again. Any suggestions? I think it might be the fuel filter but not sure. I am new to diesel motors. Thanks again for any suggestions!