6x4 Diesel Gator Surging

I have been working on this gator for a while now fixed glow plug issues and have fully painted the frame to get it ready to be sold. My last task is to tackle the rough idle that is has. So when I start it up the rough idle doesn't kick in for about 10 seconds then its starts surging. I have changed the fuel filter and that did nothing. Before I spend more money on parts I do not need to replace I figured it would ask you all.

Thanks in advance!