diesel runs odd?

[benzboi300]

Hello,
my 81 300sd has been running odd. It viabrates hard, like it has a miss.
The motor mounts are good. Some days it runs smooth some days it acts up? It does not do it when its cold. once it is warmed, it will do it at idle and up to 1200rpm, then it plains out. It does never get hot it stays about 80c. also, has new fuel filters. I am not sure what to check next. I think it might be fuel related. I even put in a new fuel pressure thing in the back of the pump. When you back that spring loaded valve back it will really get to jumpin!

suggestions? please!
Chris