Do you have the diabolical stanadyne injection pump on you machine?
If so it sounds like it's on its way out, similar problems with mine last year, had the pump rebuilt, she has more power now then she has had for a long time.
Another frequent problem is crap in the fuel tank since these tanks feed from the bottom everything gets sucked up. Check your water separator on the rh side of the fuel tank just below it. There is a little mesh screen that can get plugged.
When you have the bowl off, open the fuel valve and see how fast it flows out if it's slow, the line may be plugged. Take your air hose and blow air through the vent hole in the fuel tank cap and that should send your blockage flying out.
Also after running for a while you can take the fuel tank cap off and see if there was a vacuum, if there was the vent hole is plugged.
The fuel systems on these machines wasn't the greatest